Download Hubble 2006: Science Year in Review

National Aeronautics and Space Administration
Hubble
www.nasa.gov
Hubble 2006: Science Year in Review
Science Year in Review
This book is a joint project of NASA Goddard Space Flight Center and the Space Telescope Science Institute under contract NAS5-26555.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., in cooperation with the
European Space Agency and the National Aeronautics and Space Administration.
This recent Hubble image shows bright, blue, newly formed stars whose radiation is blowing a cavity in the center of a
star-forming region in our neighboring galaxy, the Small Magellanic Cloud.
NP-2007-4-826-GSFC
Hubble 2006: Science Year in Review
Hubble
Hubble 2006: Science Year in Review
Science Year in Review
Hubble 2006: Science Year in Review
Foreward . . . . . . . . . . . . . . . . . . . . . . 5
Hubble’s History . . . . . . . . . . . . . . . . . . . 8
Observatory Design . . . . . . . . . . . . . . . . . . 17
Operating Hubble . . . . . . . . . . . . . . . . . . . 21
Hubble News . . . . . . . . . . . . . . . . . . . . 27
Science . . . . . . . . . . . . . . . . . 31
Jupiter’s New Red Oval . . . . . . . . . . . . . . . . . 33
Binaries (and More) in the Kuiper Belt . . . . . . . . . . . . 43
Cepheid Calibration . . . . . . . . . . . . . . . . . . 51
The Great Nebula in Orion . . . . . . . . . . . . . . . . 59
Transiting “Hot Jupiters” near the Galactic Center . . . . . . . . . 67
Einstein Rings: Nature’s Gravitational Lenses . . . . . . . . . . 75
A New Population of Active Galactic Nuclei . . . . . . . . . . . 83
Galaxies over the Latter Half of Cosmic Time . . . . . . . . . . 91
Outflows from Active Galactic Nuclei . . . . . . . . . . . . 101
Galactic Jets . . . . . . . . . . . . . . . . . . . . 111
Supporting Hubble: Profiles .
.
.
.
.
.
.
.
.
.
. 121
Coming Attractions . . . . . . . . . . . . . . 135
Further Reading . . . . . . . . . . . . . . . . . . . 144
Acknowledgments . . . . . . . . . . . . . . . . . . 145
The Hubble Space Telescope, sporting new solar arrays and other important but less visible new hardware, begins its separation from the
Space Shuttle Columbia following its last astronaut servicing call in March 2002. Upgrades have kept the observatory at the cutting edge
of astronomical research.
Hubble 2006: Science Year in Review
Inside
Hubble 2006: Science Year in Review
Foreword
On October 31, 2006, NASA Administrator Mike Griffin announced his decision to launch a shuttle mission in 2008 to refurbish and
upgrade the Hubble Space Telescope. This decision provides the opportunity to significantly increase Hubble’s scientific capability
and prolong its life as humankind’s most productive scientific instrument. With successful servicing, Hubble will be at its all-time
peak performance, with instruments many times more sensitive than the original set launched with the observatory in 1990.
Meanwhile, Hubble continues to produce great science. In 2006, almost 700 peer-reviewed, scientific papers were published using Hubble data—more than in any previous year. Images from the telescope continue to excite and inspire both
the layman and the scientist.
The 10 science articles selected for this year’s annual science report exemplify the range of Hubble research—from the Solar
System, across our Milky Way, and on to distant galaxies. The objects of study include a new feature on Jupiter, binaries in
the Kuiper Belt, Cepheid variable stars, the Orion Nebula, distant transiting planets, lensing galaxies, active galactic nuclei,
“red-and-dead” galaxies, and galactic outflows and jets. Each narrative strives to construct the reader’s understanding of the
topics and issues, and to place the latest research in historical, as well as scientific, context.
These essays reveal trends in the practice of astronomy. More powerful computers are permitting astronomers to study everlarger data sets, enabling the discovery of subtle effects and rare objects. (Two investigations created mosaic images that
are among the largest produced to date.) Multiwavelength data sets from ground-based telescopes, as well as other great
observatories—Spitzer and Chandra—are increasingly important for holistic interpretations of Hubble results.
This yearbook also presents profiles of 12 individuals who work with Hubble, or Hubble data, on a daily basis. They are
representative of the many students, scientists, engineers, and other professions who are proudly associated with Hubble.
Their stories collectively communicate the excitement and reward of careers related to space science and technology.
We hope you enjoy this portrait of Hubble’s successful and “sweet sixteenth” year in orbit. The best is yet to come!
Left: An exquisite recent Hubble image of the edge-on disk galaxy NGC 5866.
Pages 6-7: Opaque, dark knots of gas and dust are seen against the background of a nearby emission nebula known as NCG 281.
5
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
The Space Telescope will help solve many astronomical puzzles. The greatest
excitement, however, will come when the pictures returned from the satellite reveal
things no one in this generation of astronomers has dreamed of, phenomena that
only the next generation will be privileged to understand.
- John N. Bahcall and Lyman Spitzer, Jr., 1982, Scientific American, 247, 40.
Hubble 2006: Science Year in Review
Hubble’s History
Hubble’s remarkable mission has now spanned 16 years. During that time, it has been at the
nexus of perhaps the most exciting period of discovery in the history of astronomy. At the same
time, Hubble has offered up some of the most daunting challenges to humans working in space,
and success in meeting those challenges has been among NASA’s greatest triumphs.
Since its launch in 1990, Hubble has been visited four times by astronauts to fix, restore, and upgrade its equipment. In
nearly constant use between these servicing missions, Hubble has generated data for thousands of scientific papers, on topics ranging from discoveries of solar systems in formation, to precise measurements of the age of the universe.
The concept of a large telescope in space is as old as the space program itself. In a classified study in 1946, Lyman Spitzer
first articulated the scientific and technical rationale for space astronomy. He continued to be the champion of the dream of
a large telescope in space until it was realized. Supported by colleagues John Bahcall, George Field, and others, Spitzer was
a tireless advocate within the astronomical community, to the public, and to the Federal Government. The outcome was a
“new start” for the mission, authorized by Congress in 1977.
The technology needed for the Hubble Space Telescope was well advanced when work began. However, other serious technological and management challenges characterized the tumultuous years of Hubble’s design and manufacture. This turmoil
culminated with the tragic loss of Space Shuttle Challenger and its crew in January 1986. Finally, against the backdrop of
unrestrained anticipation by the public and the astronomical community alike, NASA launched Hubble into orbit on Space
Shuttle Discovery (STS-31) on April 24, 1990.
Astronauts train to service Hubble in a huge, water-filled tank that simulates weightlessness. The astronauts wear pressurized suits
similar to those they wear in orbit. They spend weeks doing this kind of training, and weeks in class. The astronauts also train using
virtual reality, and in a chamber that mimics space temperatures of +200 to –200º F (+93 to –93º C). Here, astronauts practice replacing
Hubble’s main computer—a task successfully accomplished during Servicing Mission 3A in December 1999.
8
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Hubble’s first few months were disastrous. Instead of returning crisp, point-like images of stars, its images showed stars surrounded by large, fuzzy halos of light. The source of the problem was traced to an error in constructing the equipment used to test
Hubble’s mirror during manufacture. Optical tests using this equipment led technicians to grind the mirror to the wrong shape,
giving it a classic case of “spherical aberration.” The mirror was perfectly smooth, but would not focus light to a single point.
Hubble was designed to be visited by astronauts. Even before launch, NASA had begun to build a second-generation camera
to replace the main camera that was launched with the telescope. Optical experts realized they could build corrective optics
into the camera to counteract the flaw in the Hubble mirror. NASA accelerated work on the Wide Field Planetary Camera 2
(WFPC2), and Hubble scientists and engineers designed a mechanical fixture called Corrective Optics Space Telecope Axial
Replacement (COSTAR) to deploy corrective optics in the light paths to the other instruments. In December 1993, astronauts
returned to Hubble and undertook an ambitious set of space walks to install the new equipment. The modifications worked
flawlessly, restoring Hubble’s image quality to nearly the original design goals.
In the decade following the first servicing mission, Hubble has treated astronomers and the public to the clearest and deepest
views of the universe—scenes of profound beauty and intellectual challenge. Thousands of astronomers have used Hubble
for boundary-breaking research in virtually all areas, from our own Solar System to the farthest depths of the expanding
universe. Three additional servicing missions in 1997, 1999, and 2002 punctuated this era, and a final mission to upgrade
and refurbish Hubble is planned for 2008.
The 1997 mission brought tremendous improvements to Hubble’s spectroscopic capabilities with the insertion of the Space
Telescope Imaging Spectrograph (STIS). STIS observations not only demonstrated that black holes are ubiquitous in the
centers of galaxies, but also showed that the black hole masses are tightly correlated with the masses of the surrounding
ancient stellar population. The 1997 mission also opened Hubble’s view to the near-infrared universe with the Near Infrared
Camera and Multi-Object Spectrometer (NICMOS). The clear views of distant galaxies provided by NICMOS have supplied
a wealth of clues to the complex physics in the early universe that led to the formation of the Milky Way.
Hubble was integrated at the Lockheed Martin Space Systems facility in Sunnyvale, California, where it appears in this pre-launch image.
It is roughly the size of a subway car, 42.5 feet long, and 14 feet wide at its widest point. A close look at this image reveals a portion of the
225 ft. of yellow handrails installed around the outside for astronauts to grip during servicing mission spacewalks.
11
Hubble 2006: Science Year in Review
The servicing mission in 1999 enhanced many of Hubble’s subsystems, including the central computer, a new solid-state
data-recording system to replace the aging magnetic tape drives, and the gyroscopes needed for pointing control. A month
prior to launch, a gyroscope failure had forced Hubble into “safe mode,” with no ability to observe astronomical targets.
When a premature loss of solid-nitrogen coolant cut short NICMOS’s operational life, NASA engineers used innovative mechanical refrigeration technology to develop an alternate way of cooling its detectors to their operating temperature of –320º F.
This cooling system was installed in 2002, and it brought the ailing instrument back to life. NICMOS has proved crucial to
observations of very distant supernovae used to measure the acceleration of the universe. The 2002 mission also introduced
Hubble’s most powerful camera, the Advanced Camera for Surveys (ACS), providing a tenfold improvement over WFPC2.
The final servicing mission in 2008 will install two new instruments, the Cosmic Origins Spectrograph (COS) and Wide Field
Camera 3 (WFC3). COS is the most sensitive ultraviolet spectrograph ever built for Hubble. The instrument will probe the cosmic
web—the large-scale structure of the universe—whose form is determined by the gravity of dark matter and is traced by the
spatial distribution of galaxies and intergalactic gas. WFC3 is a new camera sensitive across a wide range of wavelengths (colors), including infrared, visible, and ultraviolet light. It will study planets in our Solar System, the formation histories of nearby
galaxies, and early and distant galaxies beyond Hubble’s current reach. An attempt will also be made to repair the STIS. Installed
in 1997, it stopped working in 2004. When repaired, the instrument will be used for high-resolution studies in visible and ultraviolet light of both nearby star systems and distant galaxies, providing information about the motions and chemical makeup of
stars, planetary atmospheres, and other galaxies. Astronauts will also install a refurbished Fine Guidance Sensor to replace one
degrading unit of the three already onboard. Two of these sensors are routinely used to enable Hubble’s precise pointing, and the
third is available to astronomers for making accurate measurements of stellar positions.
The Hubble Space Telescope, operating at the intersection of the robotic and the human space flight programs, embodies
both the trials and triumphs of the space program. It has survived controversy, delays, and failures, and has proven to be
one of the most powerful and productive scientific tools ever developed.
Hubble wasn’t assembled in a day. Neither was this Hubble image of the face-on spiral galaxy Messier 101 (M101). Using the power of
the Advanced Camera for Surveys installed during the last servicing mission to the satellite, it is the largest and most detailed photo of a
spiral galaxy that has ever been released from Hubble. The galaxy’s portrait is actually composed of 51 individual exposures, in addition to
elements from ground-based telescope images.
12
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
The Hubble Space Telescope
Primary mirror
Hubble’s primary mirror is made of a special glass coated with aluminum and a special compound that reflects ultraviolet light. It is 2.4-m in
diameter and collects the light from stars and galaxies and reflects it to
the secondary mirror.
FGS
Hubble has three Fine Guidance Sensors on board. Two of
them are needed to point and lock the telescope on the target
and the third can be used for stellar position measurements,
also known as astrometry.
STIS
The Space Telescope Imaging Spectrograph (STIS) is currently
not operating, but is a versatile multipurpose instrument taking
full advantage of modern technology. It combines a camera with a
spectrograph and covers a wide range of wavelengths from the nearinfrared region into the ultraviolet.
COSTAR
The Corrective Optics Space Telescope Axial Replacement (COSTAR) is not really a science instrument: it is the corrective optics
package that replaced the High Speed Photometer (HSP) during
the first servicing mission. COSTAR was designed to correct the
effects of the primary mirror’s aberration.
NICMOS
The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is an instrument for near-infrared imaging and spectroscopic observations of astronomical targets. NICMOS detects
light with wavelengths from 800 to 2500 nm.
ACS
ACS is a so-called third generation Hubble instrument. Its wide field of view is
nearly twice that of Hubble’s previous workhorse camera, WFPC2. The name,
Advanced Camera for Surveys, comes from its particular ability to map relatively
large areas of the sky in great detail.
Hubble 2006: Science Year in Review
Aperture door
Hubble’s aperture door can close if necessary
to prevent light from the Sun from entering
the telescope.
Secondary mirror
Like the primary mirror, Hubble’s secondary mirror is made of
special glass coated with aluminum and a special compound
to reflect ultraviolet light. It is .33-m in diameter and reflects
the light back through a hole in the primary mirror and into
the instruments.
Solar panels
Hubble’s third set of solar arrays produces enough power to
enable all the science instruments to operate simultaneously,
thereby making Hubble even more efficient. The panels are
rigid and unlike earlier versions, do not vibrate, making it possible to perform stable, pinpoint-sharp observations.
Communication antennae
Once Hubble observes a celestial object, its onboard computers
convert the image or spectrum into long strings of numbers
that, via one of Hubble’s two high-gain antennae, are sent to one
of the satellites that form the Tracking and Data Relay Satellite
System (TDRSS).
Support systems
Essential support systems such as computers, batteries,
gyroscopes, reaction wheels, and electronics are contained in these areas.
WFPC2
WFPC2 was Hubble’s workhorse camera until the installation of ACS. It records excellent quality images through a selection of 48 color filters covering a spectral range from
far-ultraviolet to visible and near-infrared wavelengths. WFPC2 has produced most of the
stunning pictures that have been released as public outreach images over the years.
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Observatory Design
About the size and weight of a subway car, Hubble owes much of its design to the legacy of the
Cold War, being in many respects a copy of a KH-11 reconnaissance satellite. Hubble is just one
of roughly a dozen large telescopes of similar design that have been lofted into orbit—but Hubble
was designed to look up, not down.
The heart of Hubble is its 2.4-m mirror. While small by the standards of ground-based observatories, this mirror collects
about 40,000 times as much light as the human eye, and its location above the distorting effects of the Earth’s atmosphere
allows Hubble to obtain very sharp images and view wavelengths of light that do not reach the Earth’s surface.
Hubble has an optical layout known as a Ritchie-Chrétien Cassegrain design. The incoming light bounces off the primary
mirror, up to a secondary mirror, and back down through a hole in the primary mirror, where it comes to a focus on a set of
“pickoff” mirrors that guides the light to the scientific instruments. A graphite-epoxy truss provides a rigid structure for the
main optics, and a system of baffles painted flat black is mounted within the telescope to suppress stray or scattered light
from the Sun, Moon, or Earth.
Hubble is encased in a thin aluminum shell, blanketed by many thin layers of insulation to reduce temperature fluctuations.
The telescope itself is housed in the narrower top section of the tube. Most of the control electronics sit in the middle of the
telescope, where the tube widens. The middle section also houses Hubble’s four 100-pound reaction wheels. Hubble reorients itself around the sky by exchanging momentum with these spinning flywheels. Astronauts can easily access the devices
in Hubble’s midsection, and a number of these have been replaced or upgraded during servicing missions. At the back end
of the spacecraft, the “aft shroud” houses the scientific instruments, gyroscopes, star trackers, and other components. All of
The Hubble Space Telescope floats against the background of Earth after a week of repair and upgrade by Space Shuttle Columbia astronauts
in 2002. Hubble’s fourth servicing mission gave the telescope its first new instrument installed since the 1997 repair mission—the Advanced
Camera for Surveys. It has twice the field of view and records information much faster than Hubble’s Wide Field and Planetary Camera 2.
17
Hubble 2006: Science Year in Review
Workers study Hubble’s main, 8 ft. (2.4-m) mirror prior to launch. Its concave shape focuses
the light it collects to form an image, which is then examined by Hubble’s instruments.
The next great Hubble camera, the Wide Field Camera 3 (WFC3), will be the first on Hubble to provide
wide–field, high-resolution images at wavelengths from the ultraviolet to the near infrared (200-1700 nm).
the spacecraft’s interlocking shells—the light shield, forward shell, equipment section, and aft shroud—provide a benign
thermal and physical environment, cloaked in darkness, in which sensitive telescope optics and scientific instruments can
operate properly for many years. Excluding the aperture door and solar arrays, Hubble is about 43 ft long and 14 ft in diameter at its widest point. Altogether, it weighs about 25,000 pounds.
Hubble’s electrical power comes from two 25-foot-long solar panels, which are mounted like wings on the side of the observatory and rotate to point toward the Sun. Six batteries, charged by solar power when the Sun is overhead, provide power
when the Earth blocks the Sun. Astronauts replaced the solar arrays on two occasions during servicing missions. The present arrays are rigid panels of gallium arsenide cells that were originally designed for commercial communications satellites.
They are about 30% more efficient in converting sunlight to electricity than the prior arrays. When new, they generated about
5,700 W of electrical power.
18
Hubble 2006: Science Year in Review
In a single orbit around Earth, the exterior surface of Hubble
varies in temperature from –150º F to +200° F. Despite the
harsh thermal environment, the interior of Hubble is maintained within a narrow range of temperatures—in many
areas at a “comfortable room temperature”—by its thermal control system. Temperature sensors, electric heaters,
radiators, insulation inside the spacecraft and on its outer
surface, and paints that have special thermal properties all
work in concert to minimize the expansion and contraction
that could throw the telescope out of focus, and to keep
the equipment inside the spacecraft at proper operating
temperatures. In addition to guiding the telescope, the fine
guidance sensors are used to make very precise measurements of the relative positions of stars, which is essential
for estimating distances to nearby stars or masses of components of binary star systems.
The aft shroud has room for five scientific instruments.
Over the years, NASA and ESA have manufactured 12
scientific instruments for Hubble. Each new generation
of instruments has brought enormous improvements to
the scientific capabilities of the observatory through advances in technology. Many of Hubble’s discoveries with
these new instruments would have been impossible to
Hubble was designed to be serviceable. Here astronauts during Servicing Mission 3B
(March 2002) install the first of two rigid solar array panels, replacing older, less efficient,
flexible ones.
achieve with the instruments installed at launch.
19
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Operating Hubble
The Sun and Earth rise and set for Hubble two times in three hours, as the spacecraft skims the
upper atmosphere at an altitude of 360 miles. The looming Earth blocks half the sky and regularly
interrupts most observations by blocking the line of sight to the target.
Because Hubble’s slew rate from target to target is only about as fast as the minute hand on a watch, it cannot keep ahead of
this game. Nevertheless, careful scheduling keeps Hubble gathering light from stars and galaxies almost 50% of the time.
It is the job of Hubble controllers at the Space Telescope Science Institute and NASA’s Goddard Space Flight Center to
seamlessly blend science operations and spacecraft operations 24 hours a day. Scientists and engineers at the Institute
translate the research plans of astronomers into detailed sequences of commands for the internal electronics, detectors, and
mechanisms of the scientific instruments. The preparations, carried out weeks or months in advance of the observations,
also involve selecting guide stars to stabilize the telescope pointing, and specifying the exact sequence and timing of the
observations. Spacecraft controllers work together to schedule Hubble’s communication with the ground, to load commands
into the onboard computers, to manage the collection of electrical power from solar arrays and batteries, and to curate the
data in the onboard computers. The flight operations team at Goddard monitors every system on Hubble to ensure it is working properly. If not, ground controllers can intervene to remedy the problem.
Over the past year, Hubble pursued its usual wide range of scientific programs, targeting objects ranging from nearby
planets to galaxies billions of light-years away. Among the most technically challenging observations were those aimed at
a member of the Solar System, comet Schwassmann-Wachmann 3. Discovered photographically by Arnold Schwassmann
and Arno Arthur Wachmann of Hamburg Observatory on May 2, 1930, as it was passing within 6 million miles of Earth, the
Hubble has been serviced by astronauts four times. Here, in Servicing Mission 2 (February 1997) they are seen replacing a defective reaction
wheel assembly.
21
Hubble 2006: Science Year in Review
comet has a relatively short orbital period of 5.45 years. The 1995 apparition provided fireworks, as the nucleus broke into
four fragments and the comet brightened over hundredfold in a series of outbursts. This display was driven by outgassing
from volatiles in the cometary nucleus evaporated by solar heating, rather than external tidal forces imposed by the Sun or
Jupiter. In 2001, two components of Schwassmann-Wachmann 3 were still visible.
In preparation for its return in 2006, a team of astronomers led by Phillippe Lamy of Laboratoire d’Astronomie Spatiale in France
and Harold Weaver of the Johns Hopkins University, put together a proposal to use the Advanced Camera for Surveys (ACS) on
Hubble to obtain detailed multicolor images of the disintegrating nucleus. The proposal was submitted to the Institute in January
2006, along with hundreds of other Phase I proposals from around the world requesting Hubble observing time.
Flight controllers at the Goddard Space Telescope Operations Control Center located in Greenbelt, Maryland, monitor the health and safety of the Hubble
satellite around the clock. Commands are typically sent in groups to the telescope, where they are stored onboard for later execution.
22
Hubble 2006: Science Year in Review
Two important Hubble simulators are located in the large “high-bay” area of Building 29 at the Goddard Space Flight Center. The Vehicle Electrical
System Test (VEST) unit in the foreground electrically simulates the many complex components of the satellite. The High Fidelity Mechanical Simulator
located behind it is used for “fit checks” of new hardware (such as instruments or gyroscopes) and for astronaut training.
In a process used in the 13 previous cycles, proposals were carefully reviewed by a peer committee of other scientists in the
Hubble Cycle 14 time allocation process. The Lamy-Weaver proposal was awarded nine orbits. Working with scientists at the
Institute, the team then developed a Phase II proposal, which specified the exact sequence of color filters, exposure times, and
positions to catch the comet near its closest approach to Earth—four tenths of the distance between Earth and the Sun.
For most observations, Hubble locks on distant “guide” stars in its Fine Guidance Sensors to steady itself as it takes exposures. Because Schwassmann-Wachmann 3 was moving against the background stars, Hubble had to continuously reorient
itself to track the comet, changing guide stars, and sometimes relying on just a single guide star.
23
Hubble 2006: Science Year in Review
Schwassmann-Wachmann 3 proved to be unexpectedly active,
and a major outburst developed as it approached the prime
viewing zone. Consequently, the team applied to the Institute
director and received an additional allocation of his discretionary time to obtain more coverage of the unfolding events.
Hubble observations are scheduled on a weekly basis. Individual
observations are coded as a series of commands that are uplinked
and stored onboard Hubble, instructing the telescope where to
point, acquire guide stars, and initiate exposure sequences with
specific instruments. The first nine orbits of exposure time were
obtained over April 10–11, 2006. The follow-on observations
occurred on April 18–20.
The images were temporarily stored in solid-state memory
within Hubble and then downlinked via a NASA communications satellite to a ground terminal in White Sands, New Mexico.
From there, the data were transferred to Goddard Space Flight
Center in Greenbelt, Maryland, and then to the Institute in Baltimore, where the images were stored in the Hubble data archive.
At the same time, an automatic e-mail message was sent to the
Principal Investigators, informing them that the images were
available as fully processed images, reduced using the standard calibration pipeline, and as raw images for customized
processing, if desired.
Hubble data is transmitted to Earth through a NASA relay satellite which downlinks it to a ground station in White Sands, NM.
From there, it is forwarded to Goddard Space Flight Center for
initial processing and quality checking. Within minutes it is
sent to the Space Telescope Science Institute, where it is further processed, archived, and made available to the Principal
Investigator who successfully proposed the observation.
24
Hubble 2006: Science Year in Review
The Schwassman-Wachmann 3 observations showed that the cometary nucleus had degenerated into a chain of more than
three dozen small fragments. Hubble had provided a ringside seat—and a wealth of detail inaccessible to ground-based
telescopes—for the demise of a disintegrating comet.
Moving well beyond the Solar System, Hubble observations continue to play a crucial role in probing the cosmic flow of galaxies,
finding evidence for a mysterious dark energy that pervades the universe. Type Ia supernovae have emerged as “standard candles,”
enabling measurements of large cosmic distances and providing a basis for observational cosmology. Triggered by the disruption
of compact white dwarf stars in binary systems, these supernovae are vast explosions that, at their brightest, outshine a galaxy.
Because these events are unpredictable and relatively short lived, astronomers must adopt clever strategies to detect them and
measure their brightness variations to determine their distance. Saul Perlmutter, at Lawrence Berkeley Laboratory, and Adam Riess,
at the Institute and the Johns Hopkins University, have
been using the ACS to survey distant galaxies for suitable
supernovae. These programs require particularly intense
data analysis, because supernovae fade on timescales
of days. Typically, the wide-field ACS observations are
taken from the archive, processed, and thoroughly scrutinized for new supernovae within 24–36 hours of their
being taken. This rapid turnaround is essential to allow
follow-up observations with the Near Infrared Camera
and Multi-Object Spectrometer (NICMOS) before the
supernova fades into obscurity. The NICMOS observations, added to the observing schedule over the next
1–3 weeks as target-of-opportunity observations, map
the decline in the supernova’s brightness—its “light
curve.” The shape of that light curve allows Hubble to
Hubble provided astronomers with extraordinary views of comet 73P/Schwassmann-Wachmann 3. The fragile comet rapidly disintegrated as it approached the Sun. Hubble images uncovered many more fragments than
were reported by ground-based observers. These observations facilitate
the opportunity to study in detail the demise of a comet nucleus.
measure the distances accurately enough to reveal the
acceleration of the universe at great cosmic distances.
25
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Hubble News
Hubble observations have produced a
regular stream of news about the universe. Shown here are a few recent
highlights. Details on these topics and
many others can be found on the World
Wide Web at http://hubblesite.org.
Recently discovered supernovae in distant galaxies support the theory that an unknown, i.e., “dark” energy has
influenced the universe for at least the last 9 billion years.
Using the light from the supernovae as “standard candles,”
astronomers have measured the relative size of the universe over time and derived its expansion rate. This, in
turn, helps in describing the forces at work controlling the
expansion.
Intricate wisps of glowing gas float amid a myriad of stars in this Hubble image of the supernova remnant, N132D. The ejected material
shows that roughly 3,000 years have passed since the supernova blast. This titanic explosion took place in the Large Magellanic Cloud, a
neighboring galaxy some 160,000 light-years away.
27
Hubble 2006: Science Year in Review
(Opposite Page, Top Left) NASA’s Hubble Space Telescope has uncovered what astronomers are reporting as the dimmest stars ever seen
in any globular star cluster (shown circled in the right-hand panels).
Globular clusters are spherical concentrations of hundreds of thousands
of stars within a galaxy. The cluster NGC 6397 is one of the closest
globular clusters to Earth. Seeing the whole range of stars in this area
will yield insights into the age, origin, and evolution of the cluster.
(Left) Dark matter and normal matter have been wrenched apart by the
tremendous collision of two large clusters of galaxies. Hot gas detected by
Chandra in x-rays is seen as two pink clumps in the image and contains
most of the “normal” matter in the two clusters. The blue areas are the
regions where the dark matter is clumped—as determined by the study of
the distorted (gravitationally lensed) galaxy shapes in these areas.
(Right) Precision measurements taken over a seven-year period of the positions of core stars within the globular cluster 47 Tucanae have confirmed
that gravitational segregation by mass is occurring within the cluster. The
heavier stars are slowing down and sinking to the cluster’s core, while the
lighter stars are picking up speed and moving to its periphery.
28
Hubble 2006: Science Year in Review
980703
990705
990712
000926
020903
030329
Gamma-Ray Burst Host Galaxies
Hubble Space Telescope
NASA, ESA, A. Fruchter (STScI), and the GOSH Collaboration
(Top Right) Long-duration gamma-ray bursts are powerful flashes of high-energy
radiation that are sometimes seen coming from certain types of supernovae. If Earth
were hit by such a nearby burst, the devastation could range from destroying the
ozone in our atmosphere to triggering climate change. These images are a sampling
of the host galaxies of long-duration bursts taken by Hubble. Astronomers analyzing
these surveys have concluded that our Milky Way galaxy is an unlikely place for them
to occur. The green crosshairs pinpoint the location of the gamma-ray bursts, now
long faded away.
(Left) These dark, opaque knots of gas and dust are called “Bok globules.” They are
absorbing light in the center of the nearby emission nebula and star-forming region,
NGC 281. These images were taken with Hubble’s Advanced Camera for Surveys. NGC
281 is located nearly 9,500 light-years away in the direction of the constellation Cassiopeia.
29
Hubble 2006: Science Year in Review
STScI-PRC06-20
Hubble 2006: Science Year in Review
Science
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Jupiter’s New “Red Oval”
Imke de Pater, Philip Marcus, & Michael Wong
Jupiter has superlative visual appeal and scientific interest. Its size and intricate features—including multicolored bands and
spots—have fascinated astronomers and the public for as long as telescopes have delivered Jupiter’s disk to the eye. The
telescopic view is of cloud tops over a fathomless atmosphere, with no palpable surface beneath. The bands are jet streams,
which travel east and west at various rates. The spots are giant swirling storms, located in the turbulent boundaries between
jet streams. These storms are unlike any on Earth; they are not driven by the heat of oceans, like hurricanes, nor by unstable
weather fronts, like tornadoes. Instead, they are driven directly by Jupiter’s jet streams. Because those jet streams persist
indefinitely—unlike the transitory ocean temperatures or warm and cold fronts on Earth—the Jovian storms can persist for
decades or centuries, and perhaps much longer. Astronomers study such storms on Jupiter to test their theories of weather,
atmospheric circulation, and climate change. Sometimes Jupiter shows them an unexpected and potentially instructive
event, as when a new red spot was discovered in 2006.
No feature on Jupiter has the strong identity and established longevity of the Great Red Spot. For centuries, astronomers
have chronicled its changing position, shape, and size using the best technology of the day: sketchpads, photographs, and,
most recently, digital images from Hubble, as well as from ground-based observatories, deep-space probes, and even the
backyard telescopes of myriad amateur astronomers. Until recently, the Great Red Spot was the only one of its kind. Now,
it has a likeness: a second red spot, gliding along just to its south. This new feature is so young that its name is still unsettled—“Redspot Jr.,” “Oval BA,” and “Red Oval” are current contenders.
Unlike the Great Red Spot, whose provenance is lost in astronomical prehistory, the origin and evolution of the Red Oval
were well observed from the start. In the late 1930s, astronomers saw the white horizontal band just south of the Great
Red Spot break up into three elongated features. In the 1940s, these features contracted into three giant, counterclockwise,
Pages 30–31: Data from two Hubble instruments (the Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys) are combined in this spectacular image of the planet Saturn. The blue light encircling the pole is Saturn’s equivalent of the Earth’s aurora borealis.
Left: Jupiter’s new red oval is seen toward left of center in this Hubble image. Its more famous cousin, the Great Red Spot, is partially seen
farther to the right. The formation of the new oval, which is approximately the size of Earth, is described in this article.
33
Hubble 2006: Science Year in Review
Jupiter’s Great Red Spot and new Red Oval as observed by Hubble on April 8, 2006 (left) and April 16, 2006 (right).
swirling storms, which were named “White Ovals.” Filamentary clouds developed in each of the three gaps between the
White Ovals. Later, it became apparent that each of the filamentary clouds was associated with a clockwise storm. (This
association was difficult for scientists to make, because the Great Red Spot and the White Ovals had led them to expect that
Jupiter’s storm clouds would be bright, compact and oval, not extended and wispy.) These clockwise storms were arranged
with respect to the White Ovals as the storm C is with respect to storms A1 and A2 in the second sidebar figure on page
39, that is, each White Oval became separated from its neighboring White Oval by one of these storms.
In the following decades, the three White Ovals shared the same latitude, but they were usually widely separated in longitude. Indeed, they seemed to repel each other when they came too close (but were, in fact, repelled by one of the intervening
clockwise storms). Around 1996, a pair of White Ovals bunched up, trapping one of the clockwise storms between them.
This trio paraded eastward for months, seemingly a bound unit. In 1998, the two White Ovals in the group merged into one
storm, which in spring 2000, merged with the other remaining White Oval. Shortly after that, the solo White Oval became
notably rounder than any of its progenitors or the Great Red Spot.
34
Hubble 2006: Science Year in Review
On February 24, 2006, Filipino amateur astronomer Christopher Go alerted the Association of Lunar and Planetary Observers that the solo White Oval had changed color to red. The news swept through the community of amateur and professional
Jupiter watchers, who quickly confirmed the finding. Shortly thereafter, the Director of the Space Telescope Science Institute
approved two requests for observations by Hubble’s Advanced Camera for Surveys to obtain a sequence of images at several
wavelengths of the Great Red Spot and the new Red Oval.
Astronomers generally agree that the whiteness of Jupiter’s white features is due to ammonia ice. Surprisingly, no one knows
for certain what causes the redness of the Great Red Spot and the Red Oval. The most likely cause is a coloring agent or
“chromophore” contaminating the ice particles in the clouds.
In the 1970s, atmospheric chemists proposed that Jupiter’s red chromophore originates with phosphine gas, PH3, a colorless, flammable, poisonous gas. Astronomers had detected PH3 on Jupiter, and the Cassini spacecraft, which flew by Jupiter
September 18, 1997
July 16, 1998
FA
DE
BC
FA
BE
October 14, 1999
FA
September 2, 2000
BE
BA
Merger sequence of the three original White Ovals. In the late 1930s, these latitudes became white—completely around the planet—perhaps because
they became clouded over. Shortly after, the white strip was pinched at six points, which were labeled A to F. These pinches broadened until they created separate sections, which were labeled by their end points: FA, BC, and DE. During the 1940s, these sections shrank in longitude until they formed
ovals. In 1998, BC and DE merged, forming BE. In 2000, BE merged with FA, forming White Oval BA, which changed color in late 2005 or early 2006,
and now merits the name Red Oval.
35
Hubble 2006: Science Year in Review
892 nm - Methane
550 nm - Visible Light
330 nm - Ultraviolet
Family portrait of the Great Red Spot and the Red Oval obtained by Hubble’s Advanced Camera for Surveys on April 24, 2006. The red-green-blue composite image was constructed from individual exposures through filters at nominal wavelengths 892 nm (“red,” methane absorption), 550 nm (“green,”
visible light), and 330 nm (“blue,” ultraviolet light). The brightness of the two storms in the absorption band of methane indicates less light is lost going
in through the atmosphere to the cloud top and back out than going to the clouds surrounding the storm and back out. In other words, the tops of the
clouds in the storms are significantly higher than those of the surrounding clouds. (The black protrusion is an instrumental artifact.)
36
Hubble 2006: Science Year in Review
in late 2000, found it to be more abundant above the Great Red Spot than at other locations. In this theory, ultraviolet light
from the Sun helps convert PH3 to the chromophore, which is the red form of the phosphorous molecule P4. Other scientists
have challenged this theory, saying that phosphine is more likely to react with the methane and ammonia in Jupiter’s atmosphere to form other compounds.
More recent studies suggest that the chromophore is pure sulfur in the form of various chain- or ring-molecules, which vary
in color from red to yellow. In this theory, the source of sulfur is particles of ammonium hydrosulfide, NH4SH. Vertical winds
in the Great Red Spot and Red Oval would loft the NH4SH particles high enough for ultraviolet light to break them up, and
subsequent chemical reactions would produce the long molecules of liberated sulfur atoms.
Assuming that a chromophore is implicated in the color change from White Oval to Red Oval, there are at least three possibilities for why the change occurred—including some combinations of the three. First, the cloud top of the White Oval could
have initially been below the level of a chromophore in a higher atmospheric layer, but then the cloud top rose, permitting
mixing, interaction, contamination, and color change. Second, the storm could have become more efficient in dredging up
the coloring agent (or its antecedents) from a deep layer, increasing its abundance in the cloud tops. Third, a temperature
change at the latitude of the Ovals could have increased the abundance of the chromophore, because the chemical reactions
producing it are expected to depend strongly on the ambient temperature.
All three possibilities are consistent with a conjecture by one of the authors (PM) that Jupiter is entering a period of
climate change precipitated by the merging of the three White Ovals. “Climate” means the global distribution of temperature. Before the mergers, astronomers had observed that Jupiter’s cloud-top temperatures were nearly the same at
all latitudes, even though the Sun deposits much more heat at the equator than at the poles. This observation implies an
efficient mechanism to transport heat away from the equator, which, if blocked, could cause the temperature at the poles
to fall by 10º C or more.
37
Hubble 2006: Science Year in Review
Simulations of Merging Storms
Using computers, scientists can simulate the fluid dynamics of Jupiter’s atmosphere, including the effects of rotation, gravity, and gas
physics. One goal is to provide a qualitative understanding of merging storms.
The simplest simulation involves giant, counterclockwise, swirling storms in a row, at the same latitude, between two opposite jet
streams. In general, such storms will move with the local jet velocity, which here, midway between the jets and at the initial latitude of the
storms, is zero. If left undisturbed, this initial situation would not change: the storms would maintain their separations and not merge.
This situation is fundamentally unstable, however, because the smallest perturbation will lead to a merger. In the first panel, if the storm
A2 moves slightly upward in latitude, it is carried to the left in longitude by the jet stream above it, following the dashed path. The opposite displacement of A1 produces the opposite effect. The separation between A1 and A2 becomes less than a storm’s diameter within
weeks. After that, the A1 and A2 quickly merge into a single storm—“A12,” using the nomenclature of the White Ovals. This result is
contrary to the observations that giant, counterclockwise, swirling storms on Jupiter can persist for many decades.
If two plausible features are added to the simulations, the merger of A1 and A2 can be prevented. The first is a giant, clockwise, swirling storm C above A1 and A2, like was observed above BE and FA in the period 1996–1998. The second new feature is a bend in the
jet stream below A1 and A2, like the waves often observed in the Earth’s jet stream. Such bends have been observed corralling storms.
Simulations show that this configuration of storms and jet streams is stable for long periods of time, with the positions of the storms
oscillating around a neutral point. For example, if storm A2 is perturbed upward, the jet stream above it moves it left, as before, but then
it is carried downward by the clockwise flow around C, until the bottom jet stream moves it right again, and the upward swing of the
bend brings it back to its original position.
The position of storm C is constrained less strongly than A1 and A2. A minor perturbation, such as a collision with a rogue storm, can
dislodge it from its position. In that case, simulations show that A1 and A2 quickly come into contact and merge.
38
Hubble 2006: Science Year in Review
Numerical calculations show that the stirring action of the three large, counterclockwise, vigorously spinning storms could
easily account for the needed heat transport—but that a solitary White (or Red) Oval could not. The earliest effects of blocking the heat transport system in 1998–2000 were predicted in 2001 to start becoming visible in 2006, taking into account
a time lag for the atmospheric temperature to respond to the change in heating. Some of the more striking predictions were
large waves on the jet streams and the formation of new storms, which would be eminently detectable by Hubble. These new
features have not yet been observed, so it is an open question whether the color change of the new Red Oval was a random
local event with no wider ramifications, or whether it was a consequence of blocked heat transport and therefore, a harbinger
of more changes to come. Hubble’s watchful eye on Jupiter should help settle this question.
The Hubble Space Telescope is the premier observatory for observing unexpected phenomena on Jupiter—like the crash
with Comet Shoemaker-Levy 9 in 1994 and the color change of the White Oval observed in 2006. No other telescope offers
the same combination of synoptic perspective with the ability to capture exquisite detail over a wide range of wavelengths.
These unsurpassed qualities have allowed astronomers to witness rare events and obtain unique data records of the highest
quality, which will advance understanding of the ever changing and always intriguing Jupiter.
Page 40: The ever changing cloud patterns of the giant planet Jupiter are the subject of this article. This Hubble image shows the planet’s
trademark belts and zones of high- and low-pressure regions in crisp detail. Careful study of these features results in a better understanding of planetary atmospheres—including our own.
39
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Imke de Pater is a Professor in the Department of Astronomy and Department of Earth and Planetary Science
at the University of California at Berkeley. She started her career observing and modeling Jupiter’s synchrotron
radiation, and later, she studied the planet’s thermal radio emission. In 1994, she led a worldwide observing
campaign on the impact of comet D/Shoemaker-Levy 9 with Jupiter. Currently, she uses adaptive optics in the
infrared range to obtain high angular resolution data on a variety of planetary phenomena, including volcanoes
on Io, weather on Titan, planetary rings, and Jupiter’s new Red Oval. She and Jack Lissauer wrote the graduatelevel book, Planetary Sciences. Professor de Pater was a Carolyn Herschel Distinguished Visitor at the Space
Telescope Science Institute in 2006.
Philip Marcus is a Professor of Fluid Dynamics in the Department of Mechanical Engineering at the University
of California at Berkeley. He has a long-standing interest in the theory, modeling, and numerical simulation
of the dynamics of Jupiter’s atmosphere, including its Great Red Spot and long-term climatic change. His
other astrophysical interests include the dynamics of fluids and dust in the formation of stars and planets. His
overarching goal is to understand how the fundamental physics of turbulence, chaos, and nonlinear dynamics, which are inherent to fluid dynamics, operate in astrophysical phenomena. To complement his theoretical
and computational analyses, he works closely with laboratory experimentalists, often co-designing laboratory
experiments that test or elucidate basic astrophysical processes involving fluid flows.
Mike Wong is a Research Scientist in the Department of Astronomy at the University of California at Berkeley.
His interest in cloud-forming gases in Jupiter’s atmosphere began with analyzing data from the mass spectrometer on the Galileo probe. Later, he participated with the investigator team for the Composite Infrared
Spectrometer on Cassini in the discovery of the signature of ammonia ice in Jupiter’s thermal spectrum. With
Franck Marchis, he discovered the first moonlet binary among the Trojan asteroids, a large group of objects
that share the orbit of Jupiter around the Sun.
41
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Binaries (and More) in the Kuiper Belt
Keith Noll
The Kuiper Belt is a broad ring of small planetary objects outside Neptune’s orbit, or over 30 times Earth’s distance from the
Sun. Astronomers believe that the original planet-forming disk around the young Sun extended out into this region. But here,
the disk would not have cleared as in the interior region, where the rubble of planet building was swept up by collisions, or
cast away by gravitational perturbations as the major planets formed. For years, the Kuiper Belt was a hypothetical entity,
comprising Pluto but no other known objects. Then, starting in 1992, with the advent of a new generation of sensitive digital
detectors, astronomers began looking deeper and discovering many “trans-neptunian objects” (TNOs). The number is over
1000 in 2006, the same year the International Astronomical Union redefined Pluto as a “dwarf planet” and signified it the
prototypical large TNO.
Among the most intriguing and instructive TNOs are the gravitationally bound “trans-neptunian binaries” (TNBs), which
include an astonishingly high fraction of all TNOs. (Two TNBs are multiple systems with additional members.) The unsurpassed imaging power of the Hubble Space Telescope is playing a key role in finding, characterizing, and ultimately
understanding the enigmatic TNBs.
The first TNB was Pluto: its large satellite, Charon, was discovered in 1978. A true binary, the pair orbits a point—the center
of mass—that lies outside both bodies. In 2006, Hubble found two smaller moons outside Charon’s orbit, making this
system even more fascinatingly complex. It is remarkably well ordered: the orbits are circular and lie in a common plane.
As one possible way to explain this system, astronomers speculate that Pluto was originally single, but experienced a huge
collision early in the history of the Solar System. Debris from the collision was incorporated into the satellites, whose orbits
gradually circularized by tidal forces and evolved to their current sizes. (This is also the leading scenario for the formation
of Earth’s Moon.) This hypothetical origin, if correct, puts Pluto in the minority of known TNBs.
This is an artist’s impression of noontime on Sedna, the farthest known planetoid from the Sun. Over 8 billion miles away, the Sun is reduced
to a brilliant pinpoint of light that is 100 times brighter than the full Moon. The Sun would actually be the angular size of Saturn as seen from
Earth—far too small to be resolved with the human eye. (Illustration credit: NASA, ESA , and A. Schaller.)
43
Hubble 2006: Science Year in Review
The Pluto system is revealed to be an unexpectedly complex multiple system in this image from Hubble’s Advanced Camera for Surveys. Pluto and
Charon, the two brightest objects in the image orbit around a common center of mass that lies between them, a distinction that sets them apart from any
previously known Solar System pair, but which they share in common with the bulk of trans-neptunian binaries. The two smaller satellites, Nix (closer
to Pluto/Charon) and Hydra, were unseen in the glare of Pluto and Charon until imaged by Hubble. They are in co-planar and nearly circular orbits
around the common center of mass.
44
Hubble 2006: Science Year in Review
The second TNB was 1998 WW31. The primary object was found in 1998, and its companion was first detected in 2000.
Observations by Hubble helped determine the orbit, which has maximum physical separation of 44,000 km, high orbital
elongation (eccentricity = 0.82), and a period of 520 days. Using basic physics, astronomers can use the measured orbit size
and period to compute the total mass of 1998 WW31—one six-thousandth the mass of the Pluto/Charon system, which is too
low to have circularized the orbit of the companion at the observed separation over the age of the Solar System.
In 2003, astronomers used Hubble for the first unbiased search for TNBs. “Unbiased” means that the sample was selected
without favoring any attribute that might be related to multiplicity, in order to ensure that the findings would be a fair estimate
of the occurrence of multiplicity in the general population of TNOs. The pictures from Hubble were scanned not only for
clearly resolved companions (three were found), but also for subtle distortions of the TNO images that might be caused by
an unresolved companion. (This extension beyond the normal limits of a telescope’s resolution is possible because of the
exceptional stability of the Hubble observatory.) In this way, six more TNBs were found, for a total of 9 out of a sample of 81
TNOs investigated. The best estimate—that about 11% of TNOs are multiple systems—is a lower limit, because the search
could only have detected the subset of companions that were sufficiently bright and well separated to be found at the time
of the observations. That left many beyond the range of detection, which suggests that the true fraction of multiple systems
is substantially larger.
The discoveries have accumulated rapidly. As of 2006, a total of 34 TNBs are known, of which Hubble discovered 26. Mutual
orbits have been determined for nine systems, eight based on Hubble observations of the separation and orientation of the
components at different times. The orbits show a large diversity in diameter, eccentricity, and period. The shortest period so
far is 6.4 days for the Pluto/Charon system; 2001 QT297 has the longest period, 825 days. For observers, the key constraint
is angular separation. In the most widely separated system, 2001 QW322, the components appeared to be separated by 4 arc
seconds at the time of discovery; the smallest detected separation has come from Hubble, about 50 milliarc seconds. (At the
typical distance of a TNB, about 40 times the radius of Earth’s orbit, 1 arc second corresponds to 29,000 km.) Theoretical
studies indicate no reason to suspect a lower limit to the physical separations of TNBs, and the large-amplitude, long-period
brightness variations of one TNO—2001 QG298—suggests it is one of a potentially significant population (~10–20%) of
binaries so close they are nearly in contact.
45
Hubble 2006: Science Year in Review
Naming the Worlds Beyond Neptune
There are few issues that can spark a more spirited debate than what to name newly discovered objects in the Solar System. The naming process starts with a temporary designation by the Minor Planet Center (MPC). The MPC organizes and disseminates preliminary
observational data needed to pin down the orbit, which is essential for the long-term study of newly found objects. The temporary
designation comprises the year of discovery (four digits), the half-month interval within that year (one letter, skipping “I”), and a unique
alphanumeric code for each object found in the two-week period. This code starts with A, runs through Z (again skipping “I”), and then
repeats with an appended, subscripted number that increments each time the letter is used. For example, the temporary designation 2005
FY9 refers to the 249th—9 x 25 + 24—new object reported to the MPC in the period March 16–31, 2005.
Typically, observations over a couple of years improve knowledge of an orbit to the point that the future position of the object can be
predicted within some acceptable limits of uncertainty. At that point, the object is ready for a permanent serial number, which by convention, is written in parentheses. The number of small Solar System bodies with such good orbits is now in the hundreds of thousands.
(For example, the object 2005 FY9 is now numbered 136472.) The permanent number is a signal to astronomers that an object is now a
well-established member of the Solar System.
It is interesting to note that approximately half of the trans-neptunian objects (TNOs) given temporary designations are now “lost,”
meaning that the initial estimates of their orbits were not sufficiently accurate to permit relocating them in follow-up observations, if,
indeed, any such observations were ever attempted. These objects will not receive a permanent designation unless and until they are
rediscovered at a future date.
Once an object is permanently numbered, it is eligible for naming. The International Astronomical Union is responsible for the naming of
astronomical objects, a mandate carried out for TNOs by its Committee on Small Body Nomenclature. This group has established several
naming conventions for TNOs. All must be named for mythological characters. Objects in resonances with Neptune (the ratio of orbital
periods is equal to a ratio of two integers), like Pluto, are to be named for characters from underworld myths. Objects in low-inclination,
low-eccentricity orbits are named for characters from creation myths. Objects orbiting between Jupiter and Neptune are named for the hybrid
Centaurs. A recent extension of this convention calls for using the names of other mythological hybrid creatures for objects that cross the orbits of both Neptune and Saturn. With many of the more familiar names from Greek and Roman mythologies already in use, a rush has begun
on names from the rich mythology of the wider world; (90377) Sedna, named for the Inuit goddess of the sea, is an example of this trend.
46
Hubble 2006: Science Year in Review
This illustration shows two different kinds of orbits on vastly different scales. The orbit around the Sun of the trans-neptunian binary 1998 WW31 is shown
as the light-red ellipse, which extends beyond Neptune and Pluto into the swarm of more distant icy bodies known as the Kuiper Belt. The inset illustrates
the mutual orbit of the two components of the 1998 WW31 binary (for convenience, the position of the larger, brighter member of the pair is shown as
fixed). From the orbit, the mass of the binary pair can be derived using classical physics.
The image on the right of 1999 OJ4, taken with Hubble’s Advanced Camera for Surveys, shows that this object is, in fact, a previously unresolved pair of nearly
equal-sized objects in orbit around each other. Their small separation and faintness require Hubble’s unique combination of capabilities to see them.
For six of the nine systems with known orbits, the total mass has been determined with an uncertainty of 10% or better. Measured
masses range from a low of six-millionths the mass of the Moon for (58534) Logos/Zoe to 0.2 lunar masses for Pluto/Charon.
As further research clarifies the occurrence rate and dynamical characteristics of trans-neptutian binaries (TNBs), and as theoretical models are developed to interpret the results, we can anticipate a profound contribution to our understanding of conditions and processes in the original protoplanetary disk. Massive collisions may have formed a few of the largest multiple
systems, like Pluto and 2003 EL61.
Page 48: Hubble performed the first direct measurement of a KBO’s size by imaging the distant object known as Quaoar (presented here in an
artist’s concept). This icy world is approximately half the size of Pluto, making it one of the larger Kuiper Belt Objects recently discovered.
47
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Nevertheless, the nearly equal size of components in TNBs suggests that the vast majority cannot have formed from collisions. On the other hand, the direct, collisionless capture of a single similar-mass companion by a solo TNO is impossible
under the laws of orbital mechanics. Thus, most TNBs must have formed through multibody, gravitational encounters. For
either collisions or capture to have produced the number of binaries being found, a much denser environment than the current Kuiper Belt is required: an environment like that thought to exist in the disk surrounding the infant Sun. In other words,
the TNBs are primordial, dating back to the first few million or tens of million years after the Sun began to form. Since then,
TNBs are slowly being destroyed by rare gravitational encounters, which can also change their orbits around the Sun—but
no new TNBs can have formed. We can see hints of this slow depletion in the results of the 2003 survey. Comparing two
subsets of TNOs—those that have or have not been scattered out of the plane of the Kuiper Belt—the occurrence rate of
binarity is significantly higher for TNOs still in the plane (unperturbed orbits).
In less than 15 years, the Kuiper Belt has advanced from a hypothesis to a rich field of observational and theoretical research.
It is telling us about the earliest days of the Solar System, and about events we can only hope to observe directly in analogous structures around young nearby stars. Hubble is at the forefront of both avenues of research.
Keith Noll is an Astronomer at the Space Telescope Science Institute in Baltimore, Maryland.  He is interested in that part of the universe to which his great-grandchildren might someday travel.  For the past five
years, he has been studying the fascinating and complex objects that lie beyond Neptune with the Hubble
Space Telescope.
49
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Cepheid Calibration
Fritz Benedict
One of the sterling accomplishments of the Hubble Space Telescope has been its measurement of the “Hubble constant.” Named
after Edwin Hubble, who in 1929 discovered that the universe is expanding, the Hubble constant characterizes the present-day
expansion rate of the universe and is required to determine its age. In practice, measurement of the Hubble constant is extraordinarily difficult—Edwin Hubble’s initial estimate was off by about 600%. During most of the last century, astronomers could not
agree to within about a factor of two. Hubble Space Telescope observations in the late 1990s narrowed the range of uncertainty
to about 10%. Today, one of astronomy’s greatest challenges is to reduce the uncertainty to about 5%.
The quest for a precise estimate of the universe’s expansion rate is not just an academic exercise. Knowing its exact value
can help astronomers distinguish between different theories for “dark energy,” a mysterious pressure that counteracts gravity
and pushes galaxies apart at an accelerating rate. Two separate teams of astronomers discovered dark energy in 1998, using
Hubble and ground-based observatories to make careful measurements of the brightness of distant exploding stars. This
was one of the most surprising and profound discoveries of the 20th century. In an odd twist, pursuing this discovery has
astronomers pointing Hubble at some bright, nearby stars.
As a class, Cepheid variables are the most useful stars in the sky. They are named for their archetype, δ Cephei, the fourth
brightest star in the constellation Cepheus—although the most familiar Cepheid variable is the North Star. Cepheids vary
in brightness, with periods from a few days to about two months. They moved to center stage in astronomy in 1908, when
Henrietta Leavitt discovered a mathematical relationship between their period and their intrinsic brightness. Since then,
astronomers have employed Cepheids as “standard candles” for measuring cosmic distances.
Present among the thousands of stars seen by Hubble in this close-up of our neighboring galaxy, the Large Magellanic Cloud, are a few
whose brightness varies rhythmically over a period of days to weeks. These variable stars provide an important key to unlocking the correct
scale of the universe, as this article explains.
51
Hubble 2006: Science Year in Review
The apparent brightness of a light source varies inversely as the square of its distance. In other words, if the distance between an observer and a light source is doubled, the light source will appear four times as faint to the observer. Astronomers
can use this inverse square law to estimate distances. The difficulty lies in making the first step: finding a good standard
candle—a class of sources that vary in apparent brightness only due to changes in distance.
For years after Leavitt’s discovery, astronomers could only use Cepheids as uncalibrated standard candles, because no Cepheid had been assigned an accurate distance until fairly recently. Even so, Cepheids were immediately useful for measuring
relative distance. For example, in the early 1920s, when Edwin Hubble discovered Cepheids in the Andromeda nebula that
were about 100 times fainter than their counterparts in the Small Magellanic Clouds, he could infer that they were 10 times
farther away—and far beyond the most generous estimates of the limits of our Milky Way galaxy at that time. That result
ended the debate about whether the Milky Way constituted the entire universe, or was just one of many galaxies. Henceforth,
the proper title for Andromeda was “galaxy,” not “nebula.”
Because Cepheids are such good standard candles—the stars are bright and Leavitt’s relation is quite exact—astronomers
strive to calibrate them to the highest possible accuracy, and to investigate any possible sources of error when using them
to measure distances.
The absolute calibration of Cepheids calls for triangulating the distances to as many nearby Cepheids as possible. Astronomical triangulation involves observing a star at different times of the year, and measuring its displacement relative to
more distant stars. Its apparent position changes because we view it at different angles as Earth moves around the Sun.
The apparent displacement of a star when viewed from opposite sides of Earth’s orbit is its “annual parallax.” When a star
is 1 parsec (3.26 light-years) away, its annual parallax is 1 arc second. (One arc second is roughly the diameter of a dime,
viewed from a distance of two miles.)
With its superb resolving power, Hubble can resolve individual stars in distant galaxies. In the magnificent spiral galaxy NGC 3370, 98 million
light-years away, individual Cepheid variable stars were identified and studied.
52
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Henrietta Leavitt: Timeless Contributions
Sometimes a great advance in science is achieved through selfless work performed by a modest person not striving for recognition on the
stage of history. Such was the contribution of Henrietta Leavitt, one of many female “computers” working at the Harvard College Observatory for small hourly wages. Her years of patiently studying astronomical photographs—from 1893 until her death in 1921—culminated in
finding the missing link for measuring cosmic distance: the period-luminosity relation of Cepheid variable stars.
Leavitt’s job at the observatory was to measure the brightness of stars on photographic plates from Harvard’s telescopes in Massachusetts and abroad. Her product was a record of results comprising the identity of each star based on its position, and the star’s brightness
on the date of the observation, which she determined by comparing the star’s tiny photographic image with those of other stars. She
performed this exacting task year after year in pursuit of various projects for Edward Pickering, the observatory director.
Leavitt was asked to take special note of any stars that varied in brightness, which a small fraction do, although the various reasons for
such fluctuations were not known at the time. Until she began finding them on plates from Peru of the Small Magellanic Cloud, all variable stars were at unknown distances, beyond the reach of triangulation. Even though the distance to the Small Magellanic Cloud was
also unknown, it could be assumed that all the variable stars found there were at approximately the same distance. In that case, all these
stars had the same (unknown) ratio of apparent brightness to true brightness. Therefore, observed ratios of apparent brightness between
the stars could be interpreted as ratios of true brightness, independent of the observer.
At the end of her report in the Annals of the Astronomical Observatory of Harvard College, Leavitt wrote of 16 particular stars, “It is worthy
of notice that in Table VI the brighter variables have the longer periods.” These were the Cepheid variable stars, and she had found that
the brighter the Cepheid, the longer the period of its variation. What a discovery! Astronomers have relied on it ever since to measure
distances to remote galaxies.
Once the distance to a few nearby Cepheids was measured by parallax—making the relationship between period and intrinsic brightness
absolute—astronomers began using it as a technique for estimating true astronomical distances. This continues today, probing ever
deeper into the universe as larger and larger telescopes find Cepheid variable stars in increasingly remote galaxies. Without Cepheids,
the history of astronomy would be very different, and legendary names like “Hubble” might be far less well known today.
54
Hubble 2006: Science Year in Review
The Small Magellanic Cloud is an irregularly-shaped galactic
neighbor of our Milky Way galaxy. Careful study over time of
hundreds of individual stars in this system led Henrietta Leavitt
to discover some whose variability could be related mathematically to their brightness. (Photo credit: F. Winkler/Middlebury
College, the MCELS Team, and NOAO/AURA/NSF.)
Henrietta Leavitt (Photo credit: Harvard College Observatory.)
Hubble 2006: Science Year in Review
Hubble measured the distances to 10 nearby Cepheid variable stars. Using the distances, astronomers could correct the apparent brightness of
these stars to the values that would be observed if they all were located
at the standard distance of 10 parsecs (blue dots). The best fit to the
corrected data is the new calibration of the Cepheid standard candle,
the best mathematical estimate of the relationship between period and
intrinsic brightness (upper blue line).
5000
2000
1000
500
200
100
Using the infrared camera on Hubble, astronomers measured the apparent brightness and periods of 13 Cepheids in the Large Magellanic
Cloud (red dots) and obtained a best fit to the data (red line). In this case,
the Cepheids are all at about the same, large distance from the observer.
When the Cepheid calibration curve is displaced downward by a reduction in brightness by a factor of 24.4 million, it almost exactly coincides
with the data from the Large Magellanic Cloud. Therefore, the Cloud is
the square root of that factor, or 4,940 times farther away than the standard distance of 10 parsecs, or 49,400 parsecs distant.
50
20
10
5
Apparent Brightness
2
1
0.5
0.2
0.1
0.05
0.02
x
1
The Fine Guidance Sensors on Hubble are the most accurate
(4940)2
instruments available to astronomers for measuring the an-
0.01
nual parallax of stars, induced by the yearly motion of Earth
0.005
0.002
around the Sun. Hubble has three of these powerful instru-
0.001
ments. Two are needed to stabilize the pointing of the tele-
0.0005
scope during regular operations, but the third is available to
0.0002
astronomers. With this instrument, observers can measure
0.0001
0.00005
the positions of stars in its wide field of view with an accu-
0.00002
racy exceeding 0.001 arc second, or the size of a dime seen
0.00001
from a distance of about 2,000 miles.
0.000005
0.000002
Before Hubble, very few nearby Cepheids had been assigned
0.000001
good distances. Now, Hubble has triangulated the distances
to 10 nearby Cepheids with an accuracy better than 10%.
Period in Days
56
Hubble 2006: Science Year in Review
Combined with another recent Hubble result—a small correction of the intrinsic brightness for a Cepheid’s chemical composition—these new distance measurements have significantly improved the calibration of Cepheids as standard candles.
The new Cepheid calibration was checked on two galaxies with distances well known by other means, because of special
circumstances. The first test was the Large Magellanic Cloud. (The Large and Small Magellanic Clouds are dwarf galaxies
orbiting the Milky Way.) The second was the galaxy NGC 4258, at 100 times greater distance. In both cases, the two independent distance estimates agreed to 2–3%.
Today, after a century of key contributions to science, Henrietta Leavitt’s Cepheid variable stars remain on the forefront of
astronomical research. They are the main link between the bedrock—measuring distances by geometric triangulation—and
other techniques for estimating cosmological distances to investigate the extent and structure of the universe itself. Indeed,
a key reason for building the Hubble Space Telescope originally was to measure Cepheid variable stars in galaxies at
distances of tens of millions of parsecs, which it achieved with outstanding success—laying the groundwork for today’s
understanding of the age of the universe and the mysterious dark energy. There can be no remaining doubt that Cepheids in
distant galaxies can be used with confidence to yield precise distances.
Fritz Benedict is a member of the Hubble Astrometry Science Team. His scientific interests have centered on
high precision parallaxes and the astrometric detection of low-mass companions to stars. He looks forward to
working with data from the Space Interferometry Mission, looking for the astrometric signatures of Earth-sized
planets around nearby stars. Fritz sails a Catalina 22 on Lake Travis outside Austin, Texas, walks his dog, reads
science fiction, and maintains a home with Ann, his wife of 39 years.
57
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
The Great Nebula in Orion
Massimo Robberto
The Orion Nebula is an opalescent gem in the northern winter sky—an irregular, pale-green swatch encrusted with blue stars
shining like diamonds. It is one of the nearest and most observable regions of star formation. Recently, Hubble obtained
a new portrait of this beautiful object. The largest mosaic Hubble image to date, it is truly a work of art, as well as science.
Astronomers are studying this image to better understand the formation of stars and planetary systems like the Solar System.
Of particular interest are the few very massive stars—blue, hot, and short-lived—that dominate the scene in Orion, shaping
the environment and disrupting planet-forming disks around the cooler, longer-lived, and lower-mass, yellow and red stars
like the Sun. The Orion Nebula is a special opportunity to observe the general context in which the Solar System formed.
Astronomers have a standard theory that describes how a star like the Sun and its associated planets form. The process
starts when a high-density region within a large interstellar cloud of molecular gas collapses under the weight of its own
gravity. At the center of the collapsing region, material accumulates and grows into a young star. Around it, a disk of
orbiting gas and dust forms, capturing material from the surrounding cloud, and feeding it toward the center. Meanwhile,
planets are thought to form by the accretion of material swept up in the disk. Larger planetary bodies develop sufficient
gravity to capture gas for atmospheres; smaller ones just form as rocky spheres. Over time, the mother cloud of gas and
dust dissipates. Some of the material is incorporated into the central star and forming planets. The remaining fraction is
pushed away from the system by the outward pressure of light and by a wind of atoms ejected from the surface of stars.
This clearing wind comes from the central star and possibly other stars nearby, which may later move away. A variety of
observational evidence supports this standard theory, including protostars deeply embedded in molecular clouds, disks
around most or all stars at some period in their youth, and the co-planar alignment of the planetary orbits in the Solar
System, which astronomers believe is a fossil of the original disk.
The Orion Nebula is “only” about 1,600 light-years away (9,300 trillion miles!) At that distance, Hubble can resolve features as small as the
radius of Uranus’s orbit or about one five-hundredth of the typical distance between stars in the Nebula.
59
Hubble 2006: Science Year in Review
Astronomers have little information about the wider context in which the young Solar System developed. Nevertheless,
under the reasonable assumption that the Orion Nebula is a typical star-forming region, it presents astronomers with a valuable laboratory for observing star and planetary system formation in a controlled—or at least understandable—setting. The
exact correspondences to the situation of the early Solar System may be uncertain, but the phenomena in Orion are clearly
relevant, and therefore, ultimately instructive.
The hot, blue stars in Orion are among the youngest and most massive stars in our galactic neighborhood. Such stars live
only a few tens of millions of years. By contrast, stars with the mass of the Sun or less have stable lifetimes ranging upwards
of 10 billion years. The hot, blue stars release huge amounts of energy as ultraviolet light and in strong stellar winds—on
scales far greater than for young low-mass stars.
The effect of hot, blue stars is devastating on the whole molecular cloud, not just locally around the star. Their intense ultraviolet light dissociates molecules into atoms and ions, and accelerates them to high speeds, creating powerful winds. As the
temperature and pressure in the ionized gas increases, a hot bubble forms inside the cloud. The bubble grows, and when it
reaches the nearest edge of the molecular cloud, the gas flows rapidly away, like champagne from an opened bottle. The Orion
Nebula is just such a great cavity in a cold molecular cloud—scoured out, cut open, and exposed to our view.
The Orion Nebula is about 15 light-years wide—about four times the distance from the Sun to the nearest star. Within the
open cavity, we can count thousands of stars, all formed within about the last million years. These stars include the whole
gamut of stellar masses, ranging from many cool, red stars, of typically tenths of a solar mass, to fewer warmer, yellow stars
like the Sun, to a very few hot, blue stars, typically of tens of solar masses. We see myriad low-mass stars at various stages
of early evolution—some still enshrouded in thinning dust, some with disks, some with jets, and some seemingly free and
clear—all situated in the environment created by the hot, blue stars. The goal of the new portrait of Orion is to identify, classify, and characterize the objects, investigate new phenomena, and ultimately draw lessons relevant to the Solar System.
To create the new portrait of the Orion Nebula, we obtained a mosaic of 2,000 exposures, using all Hubble’s imaging instruments and nine color filters, which cover wavelengths from the near ultraviolet to the near infrared. We combined images
60
Hubble 2006: Science Year in Review
Brown dwarfs in Orion. The faint red sources in this close-up image of the Orion Nebula are brown dwarfs or “failed” stars.
61
Hubble 2006: Science Year in Review
from the Advanced Camera for Surveys into a single image with 33,000 by 36,000 picture elements. These exposures are
the deepest look ever taken into the Orion Nebula—or any star-forming region; they pierce through the veil of nebular light
to reveal galaxies in the distant background.
Hubble’s high resolution enables us to separate the light of the closely packed stars in Orion. Its high stability and lightmeasuring accuracy overcome the challenge of the non-uniform brightness of nebula. In the Hubble images, we can
precisely compare the stellar signals through many filters. We can use these accurate comparisons to measure the stellar
temperature and luminosity. From these measurements, we can determine the radius, mass, age, and even the mass accretion rate of each star. This catalog of stellar properties will be the largest uniform survey of young stars ever achieved.
The Hubble images reveal dozens of candidates for brown dwarf, or “failed,” stars—including isolated singles, companions
of more massive “real” stars, and even some binary brown dwarfs. Brown dwarfs are stars with a mass less than 8% of the
solar mass. The temperature at the centers of these objects is not sufficient to start the nuclear reactions that cause larger
stars to shine. At a young age, they may burn deuterium—a heavy isotope of hydrogen—but this phase is short-lived, and
after it is over, brown dwarfs radiate only the energy released from their slow gravitational contraction. Old brown dwarfs
cool and fade from view, and unseen brown dwarfs represent an unknown fraction of the mass of the universe. In Orion, we
can estimate the fraction of brown dwarfs in the general star-formation process, which can be used to constrain their total
mass in the universe.
We also see many circumstellar disks in Orion, and we presume that planets may be growing in some or all of them. Some
disks have developed cometary features due to heating by ultraviolet light and stellar winds. Clearly, these disks are being
disrupted and dissipated—and perhaps interrupted in the process of building planets. If Earth formed in an environment like
Orion, its early days were not an easy ride.
The final products of the Orion portrait project will be catalogs of the brightness and location of stars, an atlas of diffuse
objects, and multi-epoch images of the region in nine filters. With these data, astronomers can address fundamental questions about the Orion Nebula, such as whether all stars formed at the same time or in different episodes, whether the massive
62
Hubble 2006: Science Year in Review
Images through various filters (denoted by false colors) reveal aspects of three circumstellar structures in Orion. The stars are generally brighter in
redder filters because of dust, which absorbs bluer light more strongly than redder light. A star at the center of each image appears to be surrounded
by a circumstellar disk, evidenced by dark silhouettes in the non-red filters. The hydrogen/nitrogen filters show arcs of emission from those elements,
excited by the intense ultraviolet light from the massive stars in the Nebula. In the bottom row, the hydrogen/nitrogen filter reveals a jet protruding from
the central star, perpendicular to disk plane, which is seen edge-on.
63
Hubble 2006: Science Year in Review
The Orion Nebula through History
The four “Trapezium” stars, seen at the center of this image, are the four brightest and most massive stars in the Orion Nebula.
Before the invention of the telescope, the Orion Nebula was never recognized as an extended object. Makers of star charts from
Ptolemy in the second century, to Johann Bayer in 1603, recorded it as a single fifth-magnitude star.
As a celestial object, the Orion Nebula is huge—larger than the full Moon—with many faint stars and rich nebulosity. Interestingly, Galileo missed the nebulosity when he observed the Trapezium region using the first astronomical telescope in 1610. He left it to a French lawyer,
Nicholas-Claude Fabri de Peiresc, to announce the discovery of the nebulosity the
next year. Charles Messier added Orion to
his famous catalog of nebulae in 1769, as
M42 and M43, thinking the two major features were unrelated objects.
The first to articulate the scientific significance of the Orion Nebula was Sir William
Herschel (1738–1822), who presciently
called it “an unformed fiery mist, the chaotic material of future suns.” Today, astronomers study the Orion Nebula to better
understand the processes that formed the
Sun and planets in the Solar System.
64
Hubble 2006: Science Year in Review
stars are latest to appear, and the reason they are clustered at the center. We can also learn the initial distribution of stellar
masses—a fundamental astrophysical principle—including the fraction of brown dwarfs. In these ways, the Orion project
will be a long-lasting contribution by Hubble to our understanding of how stars and planets form, providing insights to the
origin of the Solar System, Earth, and the life it bears.
Massimo Robberto received his Ph.D. in Astronomy at the University of Turin in Italy. He worked for 10 years at
the Turin Observatory, before moving to the Max Planck Institute für Astronomie in Heidelberg, and then to the
Space Telescope Science Institute. His technical interest is infrared astronomical cameras. His main scientific
interest is star formation, particularly in the Orion Nebula. Massimo leads the Hubble Treasury program on the
Orion Nebula. His free time is devoted to his wife Giuliana, four-year-old daughter Gloria, and their friends.
His hobbies include playing the baroque flute and piano, climbing, playing chess on the Internet, studying
harmony and counterpoint, and of course, stargazing.
65
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Transiting “Hot Jupiters” near the Galactic Center
Kailash C. Sahu
Hubble has found 16 new planetary candidates orbiting a variety of stars near the center of our Milky Way galaxy. These
discoveries are helpful in understanding the surprising phenomenon of “hot Jupiters” in a wider context.
In the mid-1990s, Swiss astronomers Michel Mayor and Didier Queloz regularly observed the Sun-like star 51 Pegasi for
months, measuring its speed along the line of sight. They found the speed varied periodically by about 130 miles per hour,
repeating every 4.2 days. After ruling out other possible explanations, they concluded this “wobbling” was due to an unseen
planetary companion, whose gravity was tugging on 51 Pegasi as it orbited the star. The inferred mass was like Jupiter’s, but
the orbital distance was only one twentieth the distance between the Earth and Sun! Being so close to the star, this planet is
expected to be hot, and according to conventional wisdom, such a hot Jupiter should not exist. No theory of star and planet
formation envisions enough material to build a Jupiter-size planet so close to a young star—yet here one exists. It must
have migrated inwards after forming farther out.
In the decade since the planet around 51 Pegasi was discovered, astronomers have been actively searching for more. More
than 200 extrasolar planets—10% are hot Jupiters—have been detected now, mostly through this gravitational wobble in
the star’s motion. The wobble technique is not practical at larger distances, however, because of the increasing faintness of
the stars. For this reason, no planet detected by wobble is located farther than 500 light-years from the Sun—less than 1%
of the distance across our galaxy.
“Transits” offer a technique to search for planets around more distant stars. In rare cases, when a planet’s orbital plane is
aligned with the line of sight, the planet will pass in front of its star once per orbit, blocking some of its light. Astronomers
A notable recent example of a planet transiting the face of a star was the passage of Venus across the disk of the Sun on June 8, 2004. This
image shows the planet as it nears completion of its passage. Recording the tiny drop in light that results from a planet partially blocking its
star is how Hubble can detect Jupiter-sized planets. (Image and processing: David Cortner)
67
Hubble 2006: Science Year in Review
A color composite of a small region of the Sagittarius Window Eclipsing
Extrasolar Planet Search (SWEEPS) field, which includes four candidates
for stars with planetary companions (circled). Radial velocity measurements support the existence of a planetary companion in the case of
SWEEPS–04 (bottom right). Superimposed is an artist’s conception of a
hot Jupiter, which shows tidal distortion of the planet and channeling of
the stellar wind along magnetic field lines.
68
Hubble 2006: Science Year in Review
can identify transiting planets by searching for this tiny dip in the apparent brightness of the star. With this technique, astronomers using ground-based telescopes have found a handful of planets around stars as far away as 6,000 light-years
from the Sun; more are expected from the many transit surveys now underway at observatories around the world.
Because of its unique capabilities, a transit campaign with Hubble is exceptionally powerful. Operating above the atmosphere,
free from most of the effects that make stars appear to flicker in brightness, Hubble can search farther and fainter stars for
planetary transits, detecting smaller changes in brightness than is possible from the ground. Furthermore, the ground-based
transit experiments suffer from false positives—artifacts masquerading as planetary transits—because of the inability to
completely separate the light of adjacent stars in crowded fields. Only Hubble can search for transits in crowded fields—like
the bulge at the center of our galaxy—which are the most desirable targets because of the increased efficiency of searching
many stars at once. For such fields, the high spatial resolution of Hubble is crucial for minimizing false positives.
The Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) program searched a dense star field close to the
galactic center, looking for transits by orbiting planets with periods less than about four days. The SWEEPS field contains
about 300,000 stars, about 60% of which are bright enough for Hubble to detect transits by planets the size of Jupiter. (These
stars are up to 5,000 times fainter than those that have been searched for transits from the ground.) Hubble took 530 pictures
of the star field, collecting over 100 gigabytes of data—the most data Hubble has ever obtained in a single week.
The observing team developed special software to measure the brightness variations of all the stars in the SWEEPS images,
being careful to correct all known instrumental effects, such as small variations in the focus of the telescope. To eliminate
false positives, the definite signature of a transit was sought: the light from the star must dip down slightly for a few minutes
or hours, with the same variation in two wavelength bands, and then recover at the same rate as the initial dip. A few hours
or days later, the same signature must repeat. Furthermore, these dips must be unique to one star and not occur at the same
time in any neighboring star, which would indicate cross-contamination of light.
Next Page: One-half of the SWEEPS field observed with Hubble’s Advanced Camera for Surveys. Sixteen of the 180,000 stars in the SWEEPS
field showed small dips in brightness during the week of observations. The dips suggest that a planet about the size of Jupiter passed
between us and the star.
69
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Analyzing the data with this software, astronomers found 16 cases where dips in brightness
are probably due to planetary transits. The candidate planets are typically the size of Jupiter and
have orbital periods ranging from 10 hours to
4 days. A large fraction of the host stars have
low mass, the lowest being 45% that of the Sun.
Previously, astronomers did not know if stars of
such low mass were capable of forming planetary systems; Hubble has now proved they can.
The planet candidates preferentially revolve
3FE'JMUFS
around stars abundant in elements heavier than
:FMMPX'JMUFS
.PEFM
hydrogen and helium, which confirms a previous finding from stars found with planets in the
solar neighborhood: heavy-element abundance
favors planetary formation.
Because of the faintness and crowding of the
targets, most of the SWEEPS candidates cannot
be confirmed by the wobble technique. Never-
Observed light curve of SWEEPS–11 with clear evidence of a transiting planet
with a radius of 30% larger than Jupiter. The top panel shows the full data set
plotted against the fraction of the derived orbital period. The bottom panel shows
an expanded view around the transit itself, along with the theoretical prediction
(in light blue) for the shape of a dip that would be caused by a planetary object.
The fit is excellent, and confidence in the interpretation is high. Radial velocity observations, obtained with the 8 m Very Large Telescope of the European
Southern Observatory in Chile, supports the planetary nature of SWEEPS–11.
71
Hubble 2006: Science Year in Review
theless, the wobble technique was successfully
applied to two of the brightest candidates, which
further confirmed the planetary nature of these
candidates. This strongly supports the estimate
that at least 45% of the SWEEPS candidates are
genuine planets.
A few of the planets orbit so fast that their year—the time for one complete revolution around the star—is less than 24 hours.
These ultra-short period planets occur only around stars less massive than the Sun. One possible explanation is that any planet
orbiting so close to more massive stars—which are much hotter and brighter—would get so hot that it would evaporate. If so,
a future search around younger stars should find hot Jupiters before they evaporate.
Hubble has contributed important new information on extrasolar planets. It has confirmed that conditions for planet formation are more favorable around stars with a greater abundance of heavy elements, and that planets not only form and survive
around all classes of stars, but their occurrence rate is similar all across the galaxy.
Kailash Sahu is an Associate Astronomer at the Space Telescope Science Institute. He is an instrument scientist for the Advanced Camera for Surveys. His research interests include the search for extrasolar planets
through transits and microlensing, the nature of dark matter, and gamma-ray bursts. He is a founding member
of the Probing Lensing Anomalies NETwork (PLANET) collaboration, which searches for planets using gravitational microlensing, and he is the Principal Investigator of the SWEEPS project which uses Hubble to detect
planets passing in front of stars in the galactic bulge.
The constellation of Sagittarius provides a virtual treasure chest of stars to search for possible planets. This Hubble image is of a different
but similar section of sky to the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field described in this article.
72
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Einstein Rings: Nature’s Gravitational Lenses
Leonidas Moustakas and Adam Bolton
In his General Theory of Relativity, published in 1915, Albert Einstein proposed that gravity bends the path of light. In 1936,
at the urging of an amateur scientist, he wrote a brief paper about an optical illusion due to this bending: multiple images of
one astronomical source located behind another. With near-perfect alignment, a full “Einstein ring” should appear around
the intermediate, lensing object. “Of course, there is no hope of observing this phenomenon directly,” Einstein wrote. In his
cover letter, he thanked the editor of Science for his “cooperation with the little publication, which Mister Mandl squeezed
out of me. It is of little value, but it makes the poor guy happy.”
Today, the elegant phenomenon of “strong” gravitational lensing—the case when multiple images can occur—makes many
astronomers happy. Even though such gravitational lenses are uncommon, many have been found and studied, with Hubble
playing an important role. Furthermore, lenses are proving to be much more than curiosities. When the lens and source are
galaxies, the illusion can teach us about nonluminous and hence unseen “dark matter,” hidden structure, and the processes
by which galaxies form and evolve.
The angular size of the Einstein ring is determined by the amount of mass—both stars and dark matter—enclosed within it.
By virtue of their stronger gravity, more massive foreground galaxies produce larger Einstein-ring images of galaxies in the
background. Measuring the mass of galaxies is a difficult, but fundamental, task of astronomy. Lensing, when it occurs, is
perhaps the most direct method.
Though gravitational lensing has been studied previously by Hubble and ground-based telescopes, this phenomenon has never been seen
before in such detail. The Advanced Camera for Surveys image of Abell 1689 reveals 10 times as many arcs as would be seen by a groundbased telescope. It is five times more sensitive and provides pictures that are twice as sharp as the previous Hubble cameras; it can see the
very faintest arcs with greater clarity.
75
Hubble 2006: Science Year in Review
The Optical Illusion of
Gravitational Lensing
The more-distant galaxy is the “source,”
and the intermediate galaxy is the “lens.” If
the source, lens, and observer are closely
aligned, the gravity of the lens will bend some
light rays from the source onto new paths towards the observer. To the observer, these
bent rays appear as if they originate at points
on the sky displaced from the location of
lens. Indeed, rays from multiple paths around
the lens and at different clock angles may arrive at the observer, creating multiple images
of the source. In the case of a near-perfect
alignment, it is possible for light rays to be
bent towards the observer all around the lens,
creating the appearance of a full Einstein ring,
as shown!
An Einstein ring does not unambiguously tell how mass is arranged within the lensing galaxy. Nevertheless, the arrangement
of mass is important for understanding the physical structure of the galaxy and its evolutionary history. We can solve this
problem by measuring the distribution, or “spread,” of stellar velocities within the lensing galaxy: the faster the stars move,
the more the mass must be concentrated towards the center of the galaxy, for gravity to balance centrifugal force.
76
Hubble 2006: Science Year in Review
Until recently, both diagnostic tools for studying galactic mass—lensing and velocity spread—were available only for a
few objects. Out of the fewer than 100 gravitational lenses known, only a small number were sufficiently near, or luminous enough, to allow accurate measurements of their velocity spreads. To surmount this difficulty, astronomers are now
combining two outstanding astronomical resources—the Sloan Digital Sky Survey and the Hubble Space Telescope—to
find large numbers of new, bright, gravitational lenses, which they can study with great precision from both space- and
ground-based observatories.
Begun in 1998, the Sloan survey has imaged roughly one fourth of the sky and measured the brightness and colors of millions of stars, galaxies, and quasars. Using spectroscopy, Sloan has also measured the distances to nearly a million galaxies. (The distances are calculated from the redshifts of the galaxy spectra due to the cosmological expansion of the universe.
See the sidebar on quasi-stellar object spectra in Arav’s article on active galactic nuclei [AGN] outflow.) Any two objects
that are found close together on the sky, but are located at vastly different distances, are prime candidates for gravitational
lensing. By sifting through all the Sloan spectra of large, luminous galaxies, astronomers have discovered hundreds of new,
bright candidates, which are usually massive elliptical galaxies in front of more distant, faint, star-forming galaxies.
Lens
Lensed Source
Flux
Observation
Wavelength
Sloan spectroscopy is key to finding gravitational lenses consisting of two galaxies lined up by coincidence. A difference in distance is indicated by a
difference in cosmological redshift, that is, characteristic spectral features appearing at different, redshifted wavelengths. The left panel shows a typical
spectrum that the Sloan spectroscopy might observe for a lens candidate (green). This candidate “lensing” galaxy is a giant elliptical galaxy, which
exhibits the composite spectrum of myriad typical stars—a spectrum that is well understood and can be easily modeled (middle panel, yellow). When
the model spectrum is redshifted to match the absorption lines in the observed spectrum, and then subtracted from the observed spectrum, the spectrum
of the distance source or “lensed” galaxy is revealed (blue). It shows “anomalous” emission lines at higher redshift than that of the lensing galaxy.
77
Hubble 2006: Science Year in Review
The Sloan-Hubble program has been extremely efficient in finding new Einstein rings.  Two of these are shown here (top row).  To make the study of
the rings themselves easier, astronomers create simple models of the lens galaxies and “subtract” them out of the Hubble images.  The results for these
two lenses are shown in the bottom row.
78
Hubble 2006: Science Year in Review
Finding the true gravitational lenses among the Sloan candidates demands the unrivaled sensitivity and image sharpness
of Hubble’s Advanced Camera for Surveys (ACS). By the summer of 2006, images have confirmed more than 40 new
gravitational lenses. Astronomers are studying the mass distribution in these lenses to better understand how galaxies
form and evolve.
Galaxy formation and evolution is a messy business, involving the gravitational force of both normal and dark matter, as well
as the myriad processes of normal matter—like star formation, radiation, cooling, turbulence, winds, and chemical enrichment. Adding to the complexity are collisions and mergers between galaxies, and the outflows and jets of supermassive
black holes in AGN (which are the subjects of three other articles in this book).
No models or simulations can yet take all these detailed processes into account. Nevertheless, a sufficient theoretical framework is available to make qualitative comparisons with the mass-related results of strong lensing.
From Hubble images and Sloan spectra of strong lenses, astronomers gain three types of new information. First, the sizes
of the Einstein rings provide the total masses of the lensing galaxies. Second, the images reveal the shapes of the lensing
galaxies, which are useful for guiding the modeling of mass structure. Third, the Sloan spectra provide initial estimates of
the velocity spreads. From this information, astronomers have reached a remarkable conclusion: The mass in the centers of
elliptical galaxies has a simple, apparently universal structure, one that has remained the same over most of the age of the
universe. Despite the varied, chaotic origins of these galaxies, their luminous and dark matter somehow interact to achieve
a single end-state of mass structure. Furthermore, this end-state is very different from the structure predicted for the case of
pure dark matter, which confirms the dominant role of the processes of normal matter in galaxy evolution.
The Sloan-Hubble research program on gravitational lenses will discover more lenses and investigate this growing sample in
greater detail. We expect to make a direct determination of how the ratio of total mass to luminosity in elliptical galaxies depends upon galaxy mass. The mass-to-light ratio is connected to the fraction of dark matter in the central region of a galaxy,
and holds clues about how and when galaxies formed—clues that we are still deciphering. A dependence of the ratio on galaxy mass has been seen in other types of observations, and it will be determined most accurately by gravitational lensing.
79
Hubble 2006: Science Year in Review
In the future, we expect that spectra from large ground-based telescopes will provide improved measurements of the velocity
distributions in individual lensing galaxies, which will improve the accuracy of the measured mass distributions. This will
lead to a better understanding of the relationships between the mass distribution and other properties of galaxies, such as
the luminosity and the history of star formation.
Sometimes an illusion is much more than it appears, as with Einstein’s rings.
Leonidas Moustakas is a Scientist at the Jet Propulsion Laboratory, which is operated by the California Institute of Technology. He has been involved in many searches for gravitational lenses—from
the Hubble Deep Field to the Hubble-Sloan program—and is currently searching for gravitational
lenses in all the Hubble data ever taken. He is especially excited about researching the evolution of
the structure in the central regions of galaxies, which is only possible with gravitational lenses.
Adam Bolton is currently a Postdoctoral Fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. He devotes most of his time to the study of galaxy structure and
evolution using strong gravitational lensing. The Hubble-Sloan project originally grew out of the
research for his Ph.D. thesis at the Massachusetts Institute of Technology. His work in observational
astrophysics combines dual interests in basic physics and amateur astronomy. The Hubble images
seen here represent a significant improvement over the view through his 8-inch backyard telescope.
The Sloan Lens ACS collaboration also includes Scott Burles, Leon Koopmans, and Tommaso Treu.
80
Hubble 2006: Science Year in Review
Two more examples of new lenses, as in the previous figure.  The lens on the right is truly a “cosmic bullseye,” where the more-distant source galaxy is
just about perfectly aligned with the massive lens galaxy and our view of it from Earth.  It is a textbook case of an Einstein ring.
81
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
A New Population of Active Galactic Nuclei
Amy J. Barger
Astronomers are pathfinders who explore the universe as Lewis and Clark explored the West—to discover what is there,
learn how it relates, and reveal the forces at work. Just as the pioneers charted the land and engaged the people to construct
a narrative about the new frontier, astronomers map the breadth and depth of the cosmos, counting and characterizing the
objects to determine the main elements and basic processes for the true story of the sky. In recent years, “active galactic
nuclei,” or AGN, have become a central theme in the cosmic story. From several lines of research, astronomers have learned
that AGN are major agents of change in galaxies, which are the building blocks of the universe. Three other articles in this
book discuss topics related to AGN and their roles in galaxy evolution: jets (Perlman) and outflows (Arav), and “red-anddead” galaxies (Davis & Faber). This article discusses a newly discovered population of AGN in relatively nearby galaxies.
AGN are luminous objects at the centers of galaxies. They emit staggering amounts of energy across the whole spectrum of
light—at radio, infrared, visible, ultraviolet, and x-ray wavelengths. The most luminous AGN are “quasi-stellar objects,” or
QSOs, which far outshine all the hundreds of billions of stars in their host galaxies.
The only source of energy capable of fueling AGN is the gravitational energy released by material falling towards a small,
unseen object with a mass that is millions to billions of times the mass of the Sun. AGN are observed to fluctuate in brightness on timescales of days, or even hours. From this timescale and the speed of light, astronomers can infer that the object
is indeed very small, being comparable in size to the planetary orbits around the Sun. The luminous material itself—ionized
gas—moves at very high velocities, from which astronomers infer that the gas revolves around an extremely heavy, but
unseen object: a supermassive black hole.
NASA and the European Space Agency (ESA) jointly released this image of the magnificent galaxy Messier 82 (M82). M82 is a prototypical
active galaxy, with strong multiwavelength emissions from its center. This mosaic image combines the light of x-rays from the Chandra
Space Observatory, infrared light from Spitzer, and optical wavelengths from Hubble. The galaxy is remarkable for its bright blue disk, webs
of shredded clouds, and fiery-looking plumes of glowing hydrogen blasting out of its central regions.
83
Hubble 2006: Science Year in Review
Black holes are so dense that nothing, not even light, can escape their gravitational pull. Stellar-mass black holes are
known to form from the collapse of massive stars at the end of their lives, but how supermassive black holes form—with
their masses of several million to several billion times the mass of the Sun—is still an enigma. Hubble observations of the
orbital motion of stars and gas around the nuclei of local galaxies reveal that supermassive black holes reside at the centers
of nearly all galaxies, including our own Milky Way. The supermassive black holes at the centers of local galaxies, which
must have been created during an earlier phase of active accretion, are now dormant, having run out of fuel to consume. By
taking a census of accreting supermassive black holes in the universe’s past, astronomers hope to piece together how local
supermassive black holes came to be.
The luminous portion of the AGN is an accretion disk around the black hole, which captures material from afar. The material
in the disk loses energy, probably via interaction with magnetic fields. The material spirals into the event horizon of the black
hole—the surface from which neither light nor matter can emerge—and disappears. The gravitational energy released in
this journey heats the accretion disk, producing the light of the AGN and driving the jets and outflows. In turn, the jets and
outflows are feedback mechanisms that govern the AGN: they blast interstellar gas out of the galaxy, ultimately starving the
accretion disk of new material and shutting down the AGN.
Distant supermassive black holes can only be identified while they are accreting. The first phase of the census involved
obtaining optical spectra of those distant sources whose appearances in optical images suggest that they may contain AGN.
Spectroscopy is a powerful technique that splits light into colors, just as a prism splits sunlight into the colors of a rainbow.
A spectrum is a record of the amount of light measured at every color, or wavelength, which can show emission or absorption features that astronomers can use to identify sources as AGN and to measure their distances. The general expansion
of the universe stretches the light waves emitted from distant sources during their journey to us. This stretching causes the
light waves to become longer, and hence, to appear redder. This stretching is referred to as the “cosmological redshift,” and
once we assume a cosmological model, it tells us the distance to an object and its age. (See the sidebar on QSO spectra in
Arav’s article on AGN outflow.)
84
Hubble 2006: Science Year in Review
Chandra x-ray and ground-based optical images of the Hubble Deep Field–North region. The Hubble Deep Field–North was originally a 10-day exposure by Hubble of a tiny region of sky located in the Big Dipper. The field has since been intensively observed at a number of different wavelengths using
many different telescopes. These new images cover a much wider area than the original Hubble images. The left image is a composite of three x-ray
energy ranges. Most of the sources in the x-ray image are active galactic nuclei powered by accretion onto supermassive black holes. The right image is
a composite of three different optical images observed with the 8.2-m Subaru telescope on Mauna Kea in Hawaii. There are many more sources visible
in the optical image, because star-forming galaxies are far more common in the universe than active galactic nuclei.
The first phase of the AGN census, at optical wavelengths, found that the universe went through a “QSO era” a few billion
years after the Big Bang, when QSOs (the most luminous AGN) were much more common than they are today. After that era,
the number densities of such extremely luminous, accreting, supermassive black holes dropped off dramatically. Thus, from
the optical data gathered, it seemed that most of the action took place at early times, after which the universe settled into a
sedate middle age.
85
Hubble 2006: Science Year in Review
But wait! Optical observations cannot detect AGN that are hidden by gas and dust; therefore, the optical census provided
only a partial glimpse of the demographics of these sources and their roles in galaxy evolution. Fortunately, there are ways
to detect hidden black holes, the best of which is by observing the high-energy x-rays that originate at the closest detectable
distances to the supermassive black holes, within a few multiples of the radius of the event horizon.
X-ray astronomy can only be performed from space, because Earth’s atmosphere blocks x-rays from reaching its surface.
In 1962, an x-ray telescope built by a team led by Riccardo Giacconi revealed a uniform x-ray glow coming from the sky.
(Giacconi, who won the Nobel Prize in Physics in 2002, was the first director of the Space Telescope Science Institute.)
Subsequent satellite observatories confirmed and extended the measurements of this glow to higher energies, but the individual galaxies that we now know produce it remained unknown for decades. A major reason for this was the same effect
that reduced the count of optically identified AGN: the cocooning of many supermassive black holes by gas and dust. Light
emitted in the AGN accretion process at wavelengths between infrared and low-energy x-rays are absorbed by the surrounding gas and dust, making the sources too difficult to identify at those wavelengths.
Chandra x-ray image with two optical counterparts from
Hubble imaging. The image to the left again shows the deep
Chandra image of the Hubble Deep Field–North region. The
optical counterparts to two of the x-ray sources identified
in the Hubble Advanced Camera for Surveys images of this
field are shown in the images to the right. The top right image shows a blue, quasi-stellar object, where the emission
from the accretion onto the supermassive black hole at the
host galaxy’s center outshines the host galaxy’s light. The
bottom right image shows a beautiful spiral galaxy with an
active nucleus obscured by dust and gas. If it were unobscured, the nucleus would probably be at least 10 times
brighter, swamping the light from the rest of the galaxy. This
source would not have been identified as an active galactic
nucleus without the x-ray imaging data.
86
Hubble 2006: Science Year in Review
The Chandra X-ray Observatory, launched by NASA in 1999, revolutionized studies of supermassive black holes. Not
only does Chandra’s great sensitivity detect high-energy x-rays, it also accurately pinpoints their locations, which enables
astronomers to follow up with other telescopes to learn more about the nature of the x-ray sources. For example, even if a
central black hole is obscured at optical wavelengths, once the x-ray observations show which galaxy contains it, optical
spectroscopy can be used to determine the distance to that host galaxy. Then, Hubble’s high-resolution imaging can be used
to determine whether the host galaxy is currently undergoing—or has recently undergone—a merging event with another
galaxy, which could provide new fuel for the galaxy’s AGN.
The deepest x-ray survey to date is the 2-million-second-long Chandra observation of the region of the Hubble Deep
Field–North. In a side-by-side comparison of the Hubble optical image with the Chandra x-ray image, one sees that there are
many more sources in the Hubble image. This is because most galaxies are dominated by star formation, and emission from
star formation peaks at optical wavelengths. Therefore, Hubble can easily detect star-forming galaxies. In contrast, the x-ray
emission from star formation is weak, because it is a much less energetic process than black-hole accretion, and even this
deepest of Chandra images only begins to detect star-forming galaxies. Thus, the x-ray image is dominated by luminous,
accreting, supermassive black holes, and there are far fewer of these than there are star-forming galaxies.
When astronomers looked at the optical counterparts to the Chandra x-ray sources, they found great diversity, ranging from
optically identifiable QSOs, to bright host galaxies with no emission-line AGN signature in their optical spectra, to extremely
faint sources for which it was difficult to obtain a redshift from optical spectroscopy. For these latter sources, astronomers
used Hubble optical and ground-based near-infrared images at various wavelengths to infer the redshifts.
In fact, astronomers were able to re-find all of the optically identified QSOs in the Hubble Deep Field–North using the x-ray
data, which provided a good check on the techniques. The truly exciting discovery was that the known QSOs were not the only
accreting supermassive black holes in the field. Astronomers discovered a new population of AGN that were not discovered
by optical observations alone, and, much to their surprise, these newly discovered AGN did not just form in the QSO era!
87
Hubble 2006: Science Year in Review
In fact, these AGN reside in relatively nearby galaxies. Although their behavior is not the same as the distant QSOs—QSOs
are voracious consumers, while these new sources are much more moderate—there are so many that together they account
for a substantial fraction of the total energy released by AGN over the history of the universe.
Black-hole accretion was not just a phenomenon of the distant past; the middle-aged universe was just good at hiding the
fact that it was still an active and exciting place!
Thanks to the complementary powers of the Great Observatories, we are completing the census and demography of AGN,
which are the leading agents of galactic evolution. Two hundred years after Lewis and Clark, their spirit is alive and their
methods are at work on the cosmic frontier.
Amy J. Barger is an Associate Professor of Astronomy at the University of Wisconsin–Madison. She
also holds an affiliate graduate faculty appointment at the University of Hawaii at Manoa. She is an
observational cosmologist who studies the star formation and accretion histories of the universe.
Born in Madison, Wisconsin, she earned her B.A. at the University of Wisconsin–Madison and her
Ph.D. from King’s College, University of Cambridge as a Marshall Scholar.
The rich field of galaxies seen here in a section of the image known as the Hubble Deep Field–North has also been targeted by the other NASA
great observatories, Chandra and Spitzer. Such coordinated observations enable scientists to study the relative amounts of electromagnetic
radiation emitted by these distant sources and identify active galactic nuclei.
88
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Galaxies over the Latter Half of Cosmic Time
Marc Davis and Sandra M. Faber
Galaxies are the basic systems of the universe. Like diamonds strewn across the sky, their tiny points of light mark the
cosmic landscape. They reveal its organization into superclusters of thousands of galaxies, which border the vast voids of
empty space. Galaxies make the universe interesting. Without them, the expansion of the universe would have diluted the
cosmic soup into a thin broth of sterile and boringly diffuse gas. Instead, the excess gravity of tiny seeds of matter from
the Big Bang slowed down cosmic expansion in regions that would become galaxies. The seeds drew in nearby gas, which
eventually spawned the first generation of stars. Over billions of years, supernova explosions enriched the mix with oxygen,
carbon, iron, and other elements that form only inside stars. These heavy elements are necessary to make planets such as
Earth. The local gas consumed by star formation was replenished by fresh gas falling in from outside. By these cycles of
stellar birth and death, galaxies became fertile, self-sustaining ecosystems and evolved to become more and more suited
for planets, and for life.
“Red-and-dead” galaxies
While we think this picture describes the evolution of most galaxies, for many it apparently does not. In some galaxies,
something intervenes to quench star formation by shutting off the supply of gas. This results in “red-and-dead” galaxies,
which lack the characteristic blue tinge of freshly formed young stars. Such galaxies have given birth to all the stars and
planetary systems they ever will, and for them the window of opportunity to create fresh habitats for life has closed.
The number of red-and-dead galaxies has increased dramatically over the latter half of cosmic time. What mysterious process is killing off star formation in the universe?
This new NASA Hubble Space Telescope image of the Antennae galaxies is the sharpest yet of this merging pair of galaxies. During the course
of the collision, billions of stars will be formed. The brightest and most compact of these star-birth regions are called super star clusters, visible here as clumps of blue/white stars. As this essay explains, galaxy collisions producing massive bursts of stars might possibly lead over
time to the formation of “red-and-dead” galaxies.
91
Hubble 2006: Science Year in Review
A
K
B
C
L
D
M
A
E
F
The AEGIS picture contains one billion picture elements—the equivalent of 500 highdefinition TV screens. It includes images of over 25,000 galaxies maturing to adulthood over
the last several billion years.
H
G
Currently, there are two different ideas to explain the origin of red-and-dead galaxies. The first is that
some galaxies may fall into massive clusters, where intergalactic gas at millions of degrees is too hot
to be collected. Starved of their gas supply, such galaxies would cease to form stars. An alternative
idea is that occasional collisions of galaxies may trigger massive bursts of star formation, producing
supernovae that sweep away the gas. Such collisions may also drive gas clouds toward the center
of a galaxy, feeding a supermassive black hole and producing such intense emission from the active
J
I
galactic nucleus that radiation pressure prevents other gas from falling into the galaxy.
92
Hubble 2006: Science Year in Review
O
D
H
I
N
L
T`
B
J
E
C
N
F
S
M
G
P
K
O
R
Q
Q
Hubble is the perfect tool for testing such theories. Its images can tease out fine details in the
P
S
R
T
structures of distant galaxies, showing which ones are forming stars normally, which are colliding, and which are red and dead. Active galactic nuclei (AGN)—the bright accretion disks feeding
supermassive black holes—stand out as bright centers, which are visible to great distances and far
back in time. In fact, each Hubble image is a “core drilling” through space-time, because successive
strata disclose information from increasingly earlier epochs. Knowing the distance to a galaxy is
the same as knowing its age when we see it, which allows us to reconstruct the galaxy populations
of previous eras.
93
Hubble 2006: Science Year in Review
The AEGIS survey
A new survey project called All-Wavelength Extended Groth Strip International Survey (AEGIS) has created one of the largest Hubble pictures to
date. The image is 1/6º wide by 1º long, stretching twice the width of the full
Moon. It is a mosaic of 63 tiles, each exposed through green and red filters,
to produce diagnostic color images of some 25,000 galaxies bright enough
for detailed study. The AEGIS team is counting the red-and-dead (quenched)
galaxies and searching for circumstances—like collisions, AGN, and other
factors—that may affect star formation on a galactic scale.
A special feature of AEGIS is a rich array of supporting data from other
space- and ground-based observatories—nine besides Hubble. The Chandra x-ray satellite locates black holes buried in dust clouds and invisible
to Hubble. Together, the Galaxy Evolution Explorer (ultraviolet) and Spitzer
(infrared) satellites measure all new star formation, including any starbursts
hidden by dust. The second phase of the Deep Extragalactic Evolutionary
Probe (DEEP 2) redshift survey from the Keck Observatory provides optical spectra of thousands of galaxies, from which distances, galaxy masses,
and chemical compositions can be determined. Each facility contributes key
information to create a full portrait of every object.
The AEGIS field, in the constellation Boötes, is one of the most intensively studied regions
of the sky, with deep coverage at all wavelengths from radio waves to x-rays. Such panchromatic coverage provides individual portraits of over 25,000 galaxies, showing star formation
and revealing factors that may affect it.
94
Hubble 2006: Science Year in Review
Keck DEEP 2 spectroscopic redshift survey
Palomar near-IR WIRC JK survey
Canada France DEEP 2 imaging survey
Chandra X-ray satellite
Spizter IRAC
VLA 6cm
Hubble
Hubble
Hubble
Hubble
Hubble
Spitzer/Multiband Imaging Photometer
Spitzer
Spitzer
Spitzer
Spitzer
/ MIPS
/ MIPS
/ MIPS
/ MIPS
Spitzer/Infrared Array Camera
Spitzer
Spitzer
Spitzer
Spitzer
/ IRAC
/ IRAC
/ IRAC
/ IRAC
Chandra
Chandra
Chandra
Chandra
Chandra
The galaxy CXO–J141741.9 illustrates how telescopes at different wavelengths are needed to tell the whole story of an object. The spatial detail of the
Hubble image is needed to identify the triggering collision. Spitzer images are needed to identify star formation heating the obscuring dust. Chandra
x-rays are needed to identify the quasi-stellar object.
The galaxy CXO–J141741.9 is an example of how multispectral data fit together. The color Hubble image shows a highly
disturbed object with clumps of newly formed stars. The brightest clump coincides with a bright x-ray source discovered
by Chandra. This source is a quasi-stellar object (QSO) fueled by the clouds of gas being swallowed by a massive black
hole. Ground-based data at millimeter wavelengths show an exceptionally bright starburst, emitting more energy at visible
wavelengths than the entire galaxy does, but the starburst is completely hidden by dense, overlying dust clouds. The direct
radiation of the starburst can be measured only at far-infrared and radio wavelengths. At mid-infrared wavelengths, the Multiband Imaging Photometer on Spitzer can see hot dust, both close to the QSO and surrounding the young stars. At shorter
infrared wavelengths, Spitzer’s Infrared Array Camera can also see the QSO.
95
Hubble 2006: Science Year in Review
CXO–J141741.9 is probably a pair of colliding galaxies in which perturbed gas clouds have burst into star formation and
ignited a central black hole, producing the QSO. The intense releases of energy from both processes will probably sweep
CXO–J141741.9 clean of gas after a few million years, which would mean that this galaxy is well on its way to becoming
quenched, red, and dead.
A new tool for classifying galaxies
To find more objects like CXO–J141741.9, the AEGIS team has created a new tool to classify galaxy images in an objective,
automated manner. The tool processes the brightness information in all the pixels of a galaxy’s image and decides what
kind of galaxy it is. To classify galaxies as normal or disturbed, the tool measures the degree of clumpiness in the image
and computes the center-to-edge brightness contrast ratio. It arranges the normal galaxies along a star-forming sequence.
It identifies fully quenched, red-and-dead galaxies, as well as systems that are colliding.
Preliminary results with this tool suggest that the collision rate has held steady over the last 9 billion years. If galaxy
collisions are very efficient in producing in red-and-dead galaxies, then the observed collision rate could fully account
for the observed rate of growth of the population of quenched galaxies. However, if only a fraction of galaxy collisions
result in new red-and-dead galaxies, as we have reason to believe, then starvation by hot intergalactic gas in galaxy
clusters may also be important. We expect that continuing analyses of AEGIS observations will help us resolve the true
causes of quenching.
96
Hubble 2006: Science Year in Review
Bir
th
o
f fi
rst
ga
lax
ies
14
13
12
The Milky Way before the Sun and Earth formed
11
The AEGIS team is studying other properties of galaxy formation, too.
in
s
ola
rn
eig
hb
10
orh
oo
d
QS
O
Er
a
From the new star-formation data, we find that each size of galaxy
motions determined from Keck spectra, we find that only in the past
few billion years have most disk galaxies (like our own Milky Way)
sta
rs
stopped colliding with other galaxies and settled down to form or-
old
es
t
9
of
dered rotating disks. We are using the same spectra to measure how
th
fast these galaxies synthesized heavy elements and enriched their in-
Bir
8
makes stars at a characteristic rate over cosmic time. From internal
terstellar gas to the level needed to make planetary systems. While we
7
cannot look directly into the past to see our own galaxy forming and
evolving, AEGIS data allow us to do the next-best thing—watch lookalikes of the Milky Way evolving billions of years ago, quite probably
ste
Sy
so
orm
ve
lo
de
Hubble looks out in space and back in time to observe star-formation rates in over 25,000
galaxies as they looked as long ago as half the age of the universe. Blue indicates regions
where star formation is ongoing, red where it has been quenched. Typical galaxies are displayed in three time zones from the distant past to more recent times. The numbers along
the central spine of the figure are “look-back” times in billions of years, i.e., the time light
has traveled to reach us. In each zone—8–9, 6–7, and 3–5 billion years—a gap separates blue galaxies to the left and red-and-dead to the right. The number of red-and-dead
galaxies increases with the passage of time, symbolized here by the increasing number of
examples of red-and-dead galaxies—two, four, and six—in the three time zones. (The six
red-and-dead galaxies at lowest redshift appear to be somewhat bluer than those at higher
redshift. This is an artifact of data processing. They are still obviously redder than the six
star-forming galaxies to the left.) The shapes of the blue star-forming galaxies become
more regular as material settles into orderly, rotating disks.
0
Di
no
sa
u
rs
1
Ea
rth
2
ps
ox
yg
e
na
3
tm
os
ph
ere
Lif
ef
4
nE
art
h
So
lar
5
m
bir
th
6
leading to the formation of suns and solar systems like our own.
97
Hubble 2006: Science Year in Review
Disturbed Light Distribution
colliding
blends
spheroids
disks
Orderly Light Distribution
Spatially Extended
Spatially Compact
0.8
spheroids
blends
0.6
disks
Morphological Fraction
colliding
The wide variety of galactic shapes and sizes is one of
the most striking features of the deep, high-resolution
AEGIS image. The automatic classification tool assigns
to normal looking galaxies the labels “spheroid,” “disk,”
or “blend,” corresponding to normal nearby galaxies.
The tool also measures the orderliness of the light distribution to identify colliding galaxies. Out to 9 billion
years ago, more than 90% of galaxies have normal types
and only 7% appear to be colliding. However, the number of spheroids more than doubles over the same time,
probably due to collisions that result in mergers.
0.4
0.2
0.0
2
4
6
Lookback Time in Billion Years
8
Hubble 2006: Science Year in Review
Sandra Faber is University Professor at the University of California, and Astronomer/Professor at the
University of California Observatories, UC Santa Cruz. She has used the Hubble Space Telescope
to discover black holes at the centers of galaxies and to study distant galaxy evolution. She is CoPrincipal Investigator of the DEEP 2 redshift survey and PI of the DEIMOS spectrograph, which was
used for the survey.
Marc Davis is Professor of Astronomy and Physics at UC Berkeley. He is a pioneer in redshift surveys
and the analysis of galaxy clustering. He is the Principal Investigator of the DEEP 2 redshift survey,
which is designed to study the evolution of galaxy properties and galaxy clustering as the universe
evolved. Davis is a member of the National Academy of Sciences and the American Academy of Arts
and Sciences.
99
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Outflows from Active Galactic Nuclei
Nahum Arav
Deep in the heart of nearly every large galaxy lurks a giant black hole weighing as much as a million to a billion suns. (A
black hole is a collapsed astronomical body, so dense that no light can escape from it.) In a small fraction of galaxies, interactions of gas with the black hole somehow trigger violent outflows, ejecting material from the galaxy at very high speed. The
origin of outflows, and their lasting effects on galaxies, are among the most hotly debated topics in astronomy today.
The study of these outflows dates back to 1967, when Roger Lynds, an astronomer at the National Optical Astronomy Observatory, uncovered unusual spectral features in the light of “quasi-stellar objects” or QSOs. There was intense scrutiny
on QSOs following Maarten Schmidt’s discovery, four years earlier, that these were the most distant objects known in the
universe. (See Perlman’s article on galactic jets.) Lynds discovered that in some QSOs, a large portion of their light was
absorbed and scattered by intervening gas, leading to broad absorption “troughs” in the spectrum. Spectra like this had
never been seen before in any astronomical object. We now know that these troughs are signatures of prodigious outflows
of material driven by the same unseen engines that produce the brilliant light of QSOs—supermassive black holes. Furthermore, we know QSOs are extreme members of the family of “active galactic nuclei,” or AGN, and that most galaxies harbor
supermassive black holes at their centers.
Lynds’s troughs are due to absorption by neutral and ionized atoms along the line of sight to the AGN. Today, astronomers
continue to use absorption-line spectroscopy of AGN—the perfect background light sources, with no spectral absorption
features of their own—to study these outflows.
The galaxy known as Centaurus A was one of the first galaxies identified whose core strongly emits radiation from radio to x-ray wavelengths. Complex mechanisms operate near the massive black holes at the centers of such galaxies, producing enormous jets and outflows
of material. Here, the center of the galaxy—actually thought to be two merging galaxies—is obscured by dark lanes of dust and gas.
101
Hubble 2006: Science Year in Review
This picture of a typical quasi-stellar
object, or “QSO,” was Hubble’s
100,000th observation, taken on June
22, 1996. Of the two bright, star-like
objects, the one on the left is the QSO,
and the one on the right is a star in
our own galaxy. From the cosmological redshift in its spectrum, we know
that the QSO is about a million times
farther away than the star (and also
much farther away than the pretty
spiral galaxy at the top). Because
the apparent brightness of a source
varies inversely as the square of its
distance, and because the star and
QSO appear about equally bright, we
can infer that the intrinsic brightness
of the QSO is a trillion times greater
than the star. (The four bright spikes
on each object are due to the optics of
the telescope.)
We picture a supermassive black hole as a voracious feeder on the stars and gas from its surrounding galaxy. This material
falls toward the black hole and fills an accretion disk. While we cannot detect a black hole directly, we can see the accretion
disk, which converts a portion of the gravitational energy released from in-falling material into heat and light. This electromagnetic radiation emerges as the bright light of AGN—or in the most luminous cases, QSOs, which far outshine their host
galaxies.
102
Hubble 2006: Science Year in Review
We also observe AGN ejecting material by two powerful processes: jets and outflows. We observe jets directly, as linear
features consisting of fully ionized gas moving at nearly the speed of light. We observe outflows indirectly, as absorption
features in the spectra of AGN. Outflows consist of partially ionized gas moving outward at lower speeds than jets, but still
up to one-third the speed of light. (“Partially ionized” means some bound electrons, which are required to produce spectral
lines.)
The total amount of energy released in feeding the black hole is much larger than the gravitational binding energy of the
galaxy. Jets and outflows carry this energy to the far reaches of a galaxy. While most of the light from the AGN escapes
without interacting strongly with the galaxy, the jets and outflows—because of their material nature—do interact strongly,
particularly with the interstellar gas. If only a small fraction of the released energy is imparted to the gas, it would be driven
out, altering the course of the galaxy’s physical and chemical development. (See the accompanying article on “red-anddead” galaxies by Davis and Faber.)
Observations from Hubble and other observatories have revealed that most large galaxies today harbor supermassive black
holes in the center, but only a small percentage of these are accreting large enough amounts of material to appear as active
nuclei. Observations of the distant universe reveal that the nuclei of galaxies were more active in the past. (See the accompanying article by Barger on AGN populations.) These observations are consistent, because the jets and outflows produced
by AGN are feedback processes—meaning that they can slow down the supply of material feeding the black hole, thereby
lowering the level of nuclear “activity” and shutting down the jets and outflows. We seek a better physical understanding
of AGN jets (see accompanying article by Perlman) and outflows (as in this article) in order to learn how they may have
contributed to the development of early galaxies into the nearby galaxies we see today.
Because images of QSOs reveal only point-like sources, we turn to their spectra—brightness according to wavelength—in
order to study their characteristics. Laboratory measurements allow us to identify the position and relative strength of the
spectral features caused by different atoms and ions. Using these laboratory results, we can interpret the troughs in AGN
spectra and translate them into estimates of the physical parameters of the outflow, including the speed, temperature, density, distance from the AGN, rate of mass outflow, and kinetic energy.
103
Hubble 2006: Science Year in Review
SDSS J0402-0641
LyA
z=2.432
C IV
Si IV
C III
Lya = Lyman alpha line of hydrogen
Intervening
z=2.2845
C iv = triply ionized carbon
(three electrons removed)
C iii = doubly ionized carbon
(two electrons removed)
Brightness
Si iv = triply ionized silicon
(three electrons removed)
LyA
z=2.590
SDSS J1537+5829
N v = quadruply ionized nitrogen
(four electrons removed)
C IV
Si IV
C III
LyA
NV
4000
Si IV
4500
5000
Intervening
z=1.2462
C IV
5500
6000
Observed Wavelength (Å)
104
Hubble 2006: Science Year in Review
6500
7000
Guide to a QSO spectrum
The spectrum of a QSO divides the light into different colors to measure the brightness versus wavelength and identify
diagnostic features. The top panel shows a typical QSO spectrum with no evidence of outflows. The brightness rises slowly
towards the blue. Superimposed are emission lines of hydrogen atoms, triply ionized silicon, and doubly and triply ionized
carbon (labeled in yellow), which originate on the accretion disk immediately around the supermassive black hole. The great
distance of the QSO is evident in the large redshift, z = 2.432, which tells us that the light from this QSO started its journey
towards us 11 billion years ago, or about 7 billion years before the Sun and Earth were formed. In the laboratory, the C iv
line is located at 1549 Å, but because of the redshift of this QSO, we observe it at (1 + z) 1549 = 5316 Å. At some point
along its path towards us, the QSO’s light encountered intervening clouds of gas, which produced a few absorption features
at a somewhat lower redshift, z = 2.2845. These clouds are hundreds of millions of light-years away from the QSO and not
physically related to it.
The bottom panel shows a QSO spectrum containing evidence for a massive outflow: absorption troughs due to Lyα, N v,
S iv, and C iv (labeled in blue), located on the blue side of corresponding emission features. The magnitude of the blue shift
indicates the absorbing material is moving towards us at speeds up to 6000 km per second relative to the QSO accretion
disk. These troughs are caused by outflows of material in or near the QSO that happen to be on the line of sight to the active
nucleus—the supermassive black hole and its luminous accretion disk—at the center of the unseen host galaxy. By studying the troughs in QSO spectra, we can determine physical parameters of the outflows, including temperature, distance from
the QSO, rate of mass outflow, and kinetic energy.
105
Hubble 2006: Science Year in Review
The early spectroscopic interpretations by Lynds established the significance of AGN outflows. Because the wavelengths of
the absorption troughs are close to those of the AGN emission, he could deduce that the absorbing material is associated
with the AGN. Because the trough wavelengths are also somewhat shorter (bluer) than those of the emission lines, he knew
that the material is moving toward us. Furthermore, the amount of blue shift measures the outflow velocity and the width of
the trough indicates the spread of line-of-sight speeds, which can be tens of thousands of kilometers per second. This is
much larger than the speed needed to escape the gravity of the host galaxies!
Many of the most useful absorption lines of outflows are in the ultraviolet spectral range. Before Hubble, most spectra of
outflows were obtained by observing AGN with substantial redshifts. Such observations took advantage of the cosmological redshift to move the ultraviolet features into the visible range convenient for ground-based observatories. The advent
of Hubble instigated a revolution in the study of galactic outflows, because it could observe closer and brighter AGN in the
native ultraviolet range of the spectral troughs.
While it was operating (between 1997 and 2004), the Space Telescope Imaging Spectrograph (STIS) on Hubble was the
premier instrument for observing AGN outflows. Its great spectral resolving power produced highly detailed profiles of
the absorption troughs, which allowed astronomers to separate geometrical effects from the thickness of the outflow, and
thereby to better estimate the amount of gas involved in the flow. It could obtain high-quality spectra from 1100 Å to
3200 Å in the ultraviolet, which allowed astronomers not only to target closer AGN, but also to observe many spectral lines
that previously could not be detected. Because closer AGN appear brighter, astronomers could obtain spectra of the same
objects with the new generation of x-ray observatories, including Chandra. If the Cosmic Origins Spectrograph is installed,
or if the STIS is repaired on a future servicing mission to Hubble, this line of research can resume.
The Hubble observations have addressed a list of fundamental questions about outflows, summarized as follows:
What are the speeds? We find a big spread, from a few hundred to over a hundred thousand kilometers per second. The
higher speeds may pertain to material closer to the black hole, while outflows at greater distance may be slowed down by an
increasing burden of matter as they plow through the host galaxy.
106
Hubble 2006: Science Year in Review
BRIGHTNESS
SPECTRUM
ABSORPTION TROUGH
WAVELENGTH
Best current explanation for creating the troughs observed in the spectra of active galactic nuclei. Material (reddish clouds) falls toward the supermassive black hole (black dot). As it approaches the black hole, this matter forms a bright accretion disk (yellow). The pressure of light ejects some material
from the disk (white arrows are typical trajectories). Outflowing atoms and ions located at the blue portions of the trajectories are in the line of sight
from the telescope to the disk. Light that is absorbed by the blue material, at wavelengths characteristic of the elements and the speed of the outflow,
causes the absorption trough seen in the spectrum. Thus, interplay between geometry and moving material produces the trough seen in the inset graph.
(Figure by Daniel Zukowski.)
107
Hubble 2006: Science Year in Review
What is the material in the outflows? We find ordinary elements in various states of ionization. The relative amount of heavy elements is somewhat larger than that of the Sun. This result indicates vigorous star formation in the vicinity of the AGN, and offers
a special opportunity to study chemically enriched environments when the universe was less than 10% of its current age.
How far are the outflows from the black hole? We observe some outflows only a light-day away from the black hole—roughly
five times the distance of Pluto from the Sun. We see others at tens of thousands of light-years away—the typical size of a host
galaxy. These results are consistent with our picture that the high speed and momentum of AGN outflows carry them with continuity from the accretion disk to the far reaches of the galaxy. The travel time would be a hundred thousand to a million years.
What accelerates the outflows? Here, we are less certain, but the favored answer is the pressure of light. The great luminosity of
the AGN pushes steadily on the gas and gradually accelerates it to high speed.
Do outflows really affect the growth of the black hole and the host galaxy? Theoretically, yes. In simulations, outflows can regulate the growth of both the black hole and the host galaxy. We are, however, still some distance from demonstrating that real
AGN outflows carry enough energy and mass to play these roles. This is our main challenge in the coming decade.
Nahum Arav is a Research Professor at the Center for Astrophysics and Space Science at the University
of Colorado, Boulder. He has been researching AGN outflows for more than a decade. His work includes
observations with the Hubble Space Telescope, the Chandra X-ray Observatory, the Far Ultraviolet Spectroscopic Explorer satellite, and ground-based telescopes, as well as theoretical studies. When not researching AGN outflows, Nahum can be found in the Colorado mountains—hiking and backpacking (in
the summer), or skiing (in the winter).
108
Hubble 2006: Science Year in Review
These Hubble snapshots reveal dramatic activities within the core of the galaxy NGC 3079, where a lumpy bubble of hot gas is rising from a
cauldron of glowing matter.
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Galactic Jets
Eric Perlman
Hubble’s 1993 images of the host galaxies of “quasi-stellar objects” (QSOs) cemented the link between QSOs and active
galactic nuclei (AGN), which are central regions of galaxies that emit light strongly all across the spectrum, from radio waves
to x-rays. For years before the advent of Hubble, astronomers conjectured that AGN were accretion disks in the process of
capturing gas and feeding it to black holes—collapsed objects so dense that no light can escape their gravitational pull. In
1994, in another of its early coups, Hubble took pictures of the active galaxy M87 that revealed the AGN to be a spiral disk.
Analysis of spectral data taken from opposite sides of the disk found evidence for a strong reversal of velocity, consistent
with orbital motion under the force of gravity. The high orbital speed of the disk implied a central mass of 3 billion Suns
gathered in a volume smaller than the Solar System. This discovery lent support to the concept of AGN as bright, swirling
fields of cosmic debris, spiraling into an unseen, bottomless drain created by a supermassive black hole. (Since these early
Hubble investigations, still stronger confirmation of the existence of black holes has come from x-ray spectra of hot gas
circling close to the black-hole event horizon—the surface from which no light or matter can emerge.)
Active galaxies exhibit other energetic phenomena besides great brightness at their centers, including narrow, high-speed
jets of material (described in this article) and slower-moving outflows that cover a much larger area (see the accompanying
article by Arav).
Streaming out from the center of the galaxy M87 like a cosmic searchlight is one of nature’s most amazing phenomena, a
black-hole-powered jet of electrons and other sub-atomic particles traveling at nearly the speed of light. This article describes how Hubble
is contributing to their understanding.
111
Hubble 2006: Science Year in Review
ACS
WFPC2
3C 273
M87
Quasi-stellar object 3C 273 and the large elliptical galaxy M87 were two of the first objects to be identified with jet-like structures, as the sidebar on the next page explains.
The image on the left is a composite of two Hubble images, one by the Wide Field Planetary Camera 2 (WFPC2), and one by the Advanced Camera for Surveys (ACS). The
ACS image was taken using its coronagraph—an instrument channel that places a black disk in front of a bright object in order to view faint details nearby.
Jets and outflows have important influence on galaxy evolution because of the vast energy they transport from the AGN,
through the surrounding galaxy, and out into intergalactic space. In the process, they sweep away interstellar gas. They can
cause additional gas to be blasted away by the supernovae that occur in the aftermath of star formation, which may be instigated by the impact of jets and outflows on dense interstellar clouds. The end result of gas removal may be a transformed
galaxy—“red and dead” in the terminology of the accompanying article by Davis and Faber. Such a galaxy can form no more
stars because of lack of gas. Furthermore, fuel ceases to fall towards the black hole, which can stall the AGN. Meanwhile,
the surrounding intergalactic gas and dust is strongly heated by the vast energy ultimately deposited beyond the galaxy by
outflows and jets.
112
Hubble 2006: Science Year in Review
Galaxies, QSOs, Black Holes, and Jets
In the history of astronomy, Michigan-born Heber Curtis is best known for taking the correct side in a highly publicized 1920
debate with fellow astronomer Harlow Shapley regarding the scale of the universe and the nature of spiral nebulae. He argued
that they were galaxies beyond our own, which now we know is true. Curtis’s role in other areas of astrophysics is less well
known, yet his seminal study of jets has lasting significance. Two years before the famous debate, he noticed a linear feature
in photographs of a bright nebula in the constellation Virgo known as M87—a “curious straight ray,” he called it—aligned
with the nebula’s bright core. This was the first discovery of a galactic jet, one of nature’s most powerful phenomena.
In the late 1940s, Australian astronomers studying the sky at radio frequencies sought to identify cosmic radio sources
by timing lunar occultations—the moments that the radio signals were eclipsed by the passing Moon. In this way, they
established that M87 and the strong radio source Virgo A are, in fact, the same object. In 1962, they measured the celestial
position of 3C 273, another strong radio source in the constellation Virgo. The next year, working from Mt. Palomar, Cal
Tech astronomer Maarten Schmidt reported a star-like object at that position, and noted its jet-like feature; 3C 273 was the
first “quasar” or quasi-stellar object (QSO) to be discovered. Its distance was the greatest ever measured to that date—about
2 billion light-years—implying a prodigious energy output a hundred times brighter than the most luminous known galaxy,
and radiating as much energy per second as a large galaxy of hundreds of billions of stars.
Two years later, Carnegie astronomer Allan Sandage showed that the optical light of 3C 273 varied in brightness on
timescales of months, meaning that this immense power must emerge from a region only light-months across. This was the
first hint that the ultimate power source for these objects might be the gravitational energy released by a massive black hole
in the process of accreting matter. Black holes are bodies so dense and massive that even light itself cannot escape their
gravitational pull.
The clear similarities between M87 and 3C 273—bright radio sources, aligned jets, and sharp, bright cores—suggested
that the QSOs resided in the center of an unseen host galaxy, which was indeed revealed by Hubble in 1993, two years after
its launch.
113
Hubble 2006: Science Year in Review
Jets appear as brilliant, narrow beams—a spine with knots—shooting out from the AGN of host galaxies. The spine is the
linear organization of the light, and the knots are bright condensations—large and small—along the spine. From the apparent displacement of knots between observations taken at different times, astronomers know that the material in jets moves
at astonishing speeds, nearly up to the speed of light. Some jets are as long as a million light-years end to end, which is 10
times the diameter of our own Milky Way galaxy.
The linearity and the alignment of galactic jets with the AGN—the black hole and accretion disk—must be due to the magnetic forces experienced by charged particles in motion. While ions and electrons move freely along magnetic field lines,
they move only with great resistance across them. Because the particles and fields react against each other, the ionized gas
of the accretion disk traps the magnetic field lines and wraps them up as the disk revolves. The same principle confines the
ionized gas of jets in a twisted rope of magnetic field lines, even at great distances from the black hole.
3C 273
M87
Multispectral appearances of portions of the jets of M87 and 3C 273. These panels were assembled from data from the Very Large Array (radio waves;
red), Spitzer (infrared; yellow), Hubble (visible; green) and Chandra (x-ray; blue). Redder shades represent regions that are brighter in the infrared
and/or radio, and bluer shades represent regions that are brighter in x-rays. Such multispectral images allow astronomers to study the
spatial relationships of higher and lower energy regions in jets. For 3C 273, the quasar itself is well off the image to the left. (Image
credit: E. Perlman, Y. Uchiyama.)
114
Hubble 2006: Science Year in Review
Schematic structure of the core of an active galaxy. (Illustration credit: John Biretta.)
115
Hubble 2006: Science Year in Review
The broad, smooth spectra of jets—their distributions of intensity with wavelength—suggest that jet light is produced by
accelerated electrons spiraling along magnetic field lines. Light produced in this manner is called “synchrotron radiation,”
because it was first observed in 1947 in a laboratory device for accelerating electrons called a “synchrotron.”
Synchrotron radiation provides another diagnostic tool for studying jets: polarimetry, which is the measurement of the direction of the vibration of an electromagnetic wave (light). Synchrotron radiation is strongly polarized by the magnetic field.
By measuring the degree of polarization and its direction, astronomers can estimate the dominant direction of the magnetic
field at any point in the image of a jet. Furthermore, the regions of a jet that display stronger polarization are likely to be
regions with the more orderly magnetic field. Conversely, where the polarization is weaker, but the synchrotron radiation is
still bright, astronomers can infer less uniformity and greater disorder in the magnetic field.
Before Hubble, radio astronomers had discovered dozens of galactic jets like those of M87 and 3C 273, the first identified
QSO. Still, only five jets had been identified at optical or shorter wavelengths, and almost nothing was known about their
physical mechanisms and energy sources. In the 16 years since Hubble was launched, astronomers have produced images
of over 50 jets in radio, infrared, optical, and x-ray emissions. Of these, the historic jets of M87 and 3C 273 are still the
best-studied and most compelling examples.
Even though galactic jets are very luminous, it is maddeningly difficult to obtain physical information about them from their
light. Spectroscopy is ineffective, because the jet material is so hot that the atoms have been fully ionized—stripped of all
electrons. Therefore, jets produce no spectral lines, which astronomers customarily use to measure composition, density,
temperature, and motion along the line of sight. Nevertheless, images of jets do reveal movement, varying appearance at
different wavelengths, and sudden flare-ups. For example, between 2000 and 2005, a knot in the M87 jet increased in brightness nearly a hundredfold at both optical and x-ray wavelengths—bright enough to outshine the AGN core, located about
200 light-years away. Astronomers strive to learn what they can from such phenomena about the structures, sizes, dynamical
processes, and light production of jets.
116
Hubble 2006: Science Year in Review
M87
1999
2005
A flare in the jet of M87. The innermost 1,000 light-years of the M87 jet, as seen in 1999 and 2005. The flaring component “HST-1” is indicated by
the yellow shading. The nucleus is located near the left edge of both panels. The contrast in the bottom panel has been decreased by a factor of 10 to
emphasize the dramatic nature of the flare, which entailed a brightening by nearly a factor of 100.
Optical and x-ray emission removes energy from jets. Yet they remain luminous, which means some refueling or reacceleration process must continue to operate throughout the jet to accelerate the high-energy electrons. Astronomers do not know
what this process is. Nevertheless, they are discovering some clues that should be helpful in understanding the flow and
balance of energy in jets.
One clue is that knots appear smaller at shorter wavelengths. Because radiation at shorter wavelengths is more energetic
than at longer ones, this must mean that the regions producing higher energy radiation are smaller. For the M87 jet, astronomers have observed this trend in Hubble images through the visible range, continuing all the way to the x-ray range,
as documented in Chandra images.
117
Hubble 2006: Science Year in Review
A second clue is a recurrent pattern of optical polarization around knots. The typical knot itself shows low polarization at its
brightest point, associating magnetic disorder with increased emission. Just upstream of the knot, the polarization is seen to
peak, and the magnetic field is observed to be aligned perpendicular to the spine, suggesting a transition region—a shock
or compression—between the knot and the smooth flow into it. Downstream from the knot, the magnetic field again relaxes
parallel to the flow. This pattern is not observed at radio wavelengths, where there is much less variation in the amount and
direction of polarization near knots. This implies that the electrons that emit high-energy radiation (optical and x-ray) do
not occupy the same volume as the lower-energy electrons producing radio waves. In other words, the optical and radio
emitting electrons do not occupy the same physical space, and the higher-energy electrons occupy regions closer to the
central spine of the jet. This is consistent with the finding that the regions of particle accelerations are progressively smaller
at higher energies.
A third clue, found recently by Hubble, Chandra, and the Spitzer infrared observatory, is that jets are bluer closer to the black
hole and redder farther out. (“Bluer” means a higher ratio of shorter wavelength radiation to longer, and “redder” means
the opposite.) This pattern, seen in both M87 and 3C 273, suggests a gradual loss of energy by the jet particles as the flow
moves out from the nucleus.
Sixteen years of Hubble observations, supported by the other NASA Great Observatories in space and great radio telescopes
on the ground, have revolutionized the study of jets. Astronomers now directly observe the high-energy processes that link
emission, structure, and magnetic fields. Many puzzles remain, including the material composition of the jets, and the flow
of energy from its original source near the black hole to the distant locations where it is emitted. If NASA successfully conducts another servicing mission to Hubble and installs the waiting Cosmic Origins Spectrograph and Wide Field Camera 3,
astronomers will have powerful new tools to work on these puzzles.
118
Hubble 2006: Science Year in Review
Distance from Nucleus (arcsec)
0
20%
40%
3
2
1
0
–1
–2
–3
0
5
10
15
20
Distance from Nucleus (arcsec)
Polarization image of the M87 jet, obtained with Hubble. Red contours represent regions of high polarization, while blue colors represent regions of
low polarization (color scale shown at top). Contours are lines of constant optical brightness of the jet at visible wavelengths. Regions of maximum
polarization are usually located upstream (to the left) of the brightest point in a knot, where the magnetic field (not shown) is perpendicular to the jet.
Much lower—or no—polarization is observed at the brightest point in each knot. The magnetic field is parallel to the jet downstream of each knot’s
brightest point. (At the distance of M87, 1 arc second translates to about 250 light-years.)
Eric Perlman is an Associate Professor of Physics and Space Sciences at the Florida Institute of Technology. Over the last 10 years he has worked extensively on jets, using Hubble, the Chandra X-ray Observatory, and other telescopes. In 2000 and 2005, he was awarded five-year grants from NASA’s Long-Term
Space Astrophysics Program to pursue this work.
119
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review — Supporting Hubble
Supporting Hubble
Hubble 2006: Science Year in Review — Supporting Hubble
Outlaw
Network Administrator
Raytheon
Daria Outlaw grew up in eastern North Carolina and came to NASA with a Bachelor’s degree in Business Management and Administration from North Carolina Central University, in Durham. She joined the Hubble family in 1997 as a specialist in information technology,
with the Raytheon Corporation. She works on the Hubble local area network, providing desktop, network, and technical support. While
her specialty is Apple Macintosh support, she also has responsibility for audio-video conferencing systems. Daria works with Hubble
staff at all levels, and takes satisfaction and pleasure at how the diversity of skills, interests, and backgrounds in the project contributes
to making the perfect team.
She’s proud to work on a project that provides so much fascinating knowledge about our universe to the science community, schools,
and the population at large. “It’s very personally motivating for me to work so close to the source of all this great information that the
public desires.”
Daria has many outside interests. For the past year, she has taught a weekly computer class as a volunteer at a local women’s shelter.
She also helps out at her daughter’s high school as a parent volunteer. She is active in her church, where she works as an advisor and
activities coordinator in the children’s and youth ministries.
To maintain sanity in a busy life, Daria recommends—and makes a practice of—taking two vacations annually. “Vacations are relaxation
for the mind, body, and soul,” she likes to say. For her, the first break is for quality time with her daughter, Ebonee. The second is for
down time, for herself or with close friends. Her favorite places for holidays are tropical and warm—or Europe. Daria also enjoys exploring the countryside by motorcycle.
Page 120–121: Located about 690 light years away in the constellation of Aquarius, the Helix Nebula, NGC 7293, is the result of a complex
series of gaseous outbursts from its central star. The striated appearance of the inner rim is caused by a hot “stellar wind” of gas plowing
into colder shells of gas and dust ejected previously by the volatile star.
122
Hubble 2006: Science Year in Review — Supporting Hubble
Campbell
Operations & Institutional Services Manager
NASA Goddard Space Flight Center
Dave Campbell was born and raised in Buffalo, New York. He started at NASA in 1983, fresh out of Morgan State University with a B.S.
degree in Computer Science. He joined the Control Center Systems Branch, which is responsible for the design and implementation of
control centers for spacecraft in low Earth orbit. Dave’s first assignment was the early implementation of the Hubble operations control
center. During that time, he also earned a second B.S. degree, in Electrical Engineering from the Johns Hopkins University. Over the next
eight years, he helped develop the computer and network components that were incorporated in the ground systems of several missions
managed by Goddard, including the Cosmic Background Explorer and the Compton Gamma Ray Observatory.
“I guess I had the classic engineer attitude growing up,” Dave explains, “I loved science and math, but disliked English classes. When
I was a kid, I loved taking things apart and putting them back together again. I was fascinated by the mechanics of how things worked.
Even at the age of 12, I was extending and re-routing the telephone connections throughout the house.”
Dave completed an M.S. in Engineering Management in 1991, when he the became the Project Manager responsible for the real-time
system in the control center for the Rossi X-ray Timing Explorer. Later, he returned to the Hubble project as the Deputy Operations and
Testing Manager for the second servicing mission. Following that mission, he worked on the Earth Observing System for a few years
before returning to Hubble for a third time, as the Deputy Manager for Operations Support.
Dave is currently the manager of the office responsible for the integrity of the operational ground system that controls the Hubble spacecraft, and for the network and voice communications in the day-to-day Hubble control center.
“Looking back on things, my dad was probably the biggest influence on me toward pursuing a career in engineering. He was an engineer,
and I was very proud of him. It was guidance from my mom, though, that helped mold me into who I am today.”
Dave lives in Clarksville, Maryland, with his wife and two children. He enjoys playing tennis, bike riding with his eight-year-old son and
five-year-old daughter, swimming, and entertaining friends.
123
Hubble 2006: Science Year in Review — Supporting Hubble
Taylor
Head of the Observation Planning Branch
Space Telescope Science Institute
Denise Taylor grew up near Norfolk, Virginia. Her high school physics teacher sparked her interest in astronomy with field trips to the
local planetarium. After obtaining a Bachelor’s degree in Physics and Astronomy from the University of Virginia, Denise spent a year
teaching physics and algebra at a military school in northern Virginia. There, she learned downhill skiing and minor auto repair (thanks to
students’ practical jokes). After that, she studied stellar evolution and star formation in spiral galaxies at the University of Massachusetts
in Amherst, gaining an M.S. in Physics, an M.S. in Astronomy, and a husband (Engineering and Software Services Resource Manager,
Dave Taylor).
For four years, Denise was the Assistant Director of Elementary Physics Laboratories at Mount Holyoke College in South Hadley, Massachusetts, in an otherwise all-male physics department for an all-female college. Her love of teaching brought her to Mount Holyoke’s
SummerMath program for several summers, where she developed and taught an astronomy class for high school girls. In 1989, Denise
became the first Telescope Operations Assistant for the International Ultraviolet Explorer (IUE) at the Goddard Space Flight Center in
Greenbelt, Maryland. For over two years, she assisted astronomers in planning their IUE observations and filling data gaps.
Denise came to the Institute in 1992 as a Technical Assistant. She became expert in implementing Hubble proposals using the High
Speed Photometer, an original Hubble science instrument. She then took over all science and calibration proposals using the Fine Guidance Sensors (FGS), working closely with the Space Telescope Astrometry Team at the University of Texas in Austin. In 1994, as one of
the first Senior Program Coordinators, Denise was responsible for implementing science and calibration proposals for the FGS, Faint
Object Spectrograph, and Wide Field Planetary Camera 2. She also helped develop operational processes for analyzing bright-object
alerts on all instruments. Later, she worked on proposals for the Space Telescope Imaging Spectrograph and Near Infrared Camera and
Multi-Object Spectrometer. The year 1997 saw her move to Calibration Manager, coordinating the implementation and scheduling of all
calibrations of scientific instruments. Denise has been the Head of the Observation Planning Branch since 1999, supervising Program
Coordinators, assisting in long-range planning, and developing policies and procedures for Hubble users.
Denise’s outside interests no longer include skiing. She finds reading and quilting to be relaxing, and enjoys spending time with her
husband and son.
124
Hubble 2006: Science Year in Review — Supporting Hubble
Barcus
Launch Support Manager
NASA Goddard Space Flight Center
Jim Barcus was born in Cheverly, Maryland, just a few miles from Goddard Space Flight Center. He graduated from St. John’s College
High School and Prince George’s Community College. He has worked at Goddard since 1968 and has been a part of the Hubble family
since 1991.
As Launch Support Manager on the Hubble development project, Jim is responsible for Hubble ground and flight hardware worth over
$2.5 billion. He has performed logistical support for every Hubble-related shuttle mission—including the high-profile STS-95 mission,
which carried Senator John Glenn and tested new Hubble equipment on orbit.
Jim is affectionately known as the “Radar O’Reilly” of the Hubble program for his resourcefulness and uncanny ability to anticipate the
program’s needs. In addition to coordinating the movement of more than 400 truckloads of Hubble equipment, he sometimes orchestrates domestic and international transportation of Hubble hardware aboard C-5 military cargo planes and seafaring barges.
“As a kid, I used to lay out on the lawn during summer nights to look up at the stars and imagine what it would be like to see them up
close. Contributing to Hubble’s development, I feel as though I’ve come as close to realizing that dream as one could without becoming
an astronaut,” says Jim. “I feel a real sense of pride playing a small part in the goal of unraveling the mysteries of the universe. It’s also
gratifying to know that the technologies we develop for Hubble benefit people in other ways. For example, I understand the sensitive
instrument detectors developed for the Hubble cameras are also being used by the medical community to perform tissue analysis without
the need for surgery. This is really great!”
Outside of work, Jim is an avid softball player—still competing after 38 years of team play. He enjoys landscaping around his new home
on the water in Delaware, and relaxing at the beach with Linda and their grown children.
125
Hubble 2006: Science Year in Review — Supporting Hubble
Moy
Safing Systems Engineer
Lockheed Martin
Ed Moy began working on Hubble over 20 years ago, as a Research Engineer for Lockheed Missiles and Space Company. He helped
develop and test the pointing-control system. From 1986 until launch in 1990, he was responsible for refurbishing and integrating the
independent safe-mode computer. This serves as a critical backup to the primary operational computer in detecting and responding to
anomalous spacecraft conditions.
Ed supported the Hubble launch at the Marshall Space Flight Center, serving as Lead Pointing-Control Engineer. He was a key player
during operational verification. Following his return to Sunnyvale, California, he helped develop and test the fix for spacecraft jitter
caused by the solar arrays. He also helped improve the safe-mode design by developing and testing multiple schemes for stabilizing the
spacecraft on the Sun—without using the gyroscopes.
In 1993, Ed came to Goddard as a System Engineer for the safing subsystem. He supported all four servicing missions in that position.
This involved developing procedures for analyzing and responding to faults in advance of the missions, as well as providing real-time
support at the console during the actual missions. Ed continues to work on the design, testing, and analysis of the pointing-control
and safing subsystems of the Hubble spacecraft. He works to refine and improve control modes based on fewer than the normal
three gyroscopes—in case of failures. These designs include the one- and two-gyro science modes, and the “Kalman filter
Sun-point mode.”
Ed says a trusted graduate professor helped form his vision of pursuing a career in spacecraft design. “The Hubble operations and engineering teams always seem to be able to perform the impossible,” he says. “It is very rewarding to work daily with such a capable crew,
and to know that our work enables major astronomical discoveries.”
Ed currently resides in Elkridge, Maryland, with his wife and their child. Among his outside interests are playing basketball and pursuing
other outdoor activities. He enjoys spending his spare time with family and friends.
126
Hubble 2006: Science Year in Review — Supporting Hubble
Spencer
Administrative Assistant, Science Missions Division
Space Telescope Science Institute
Darlene Spencer was born and raised in Baltimore, Maryland. She joined the Space Telescope Science Institute in February 1989 as a
temporary employee, and was hired full time in June of the same year. She is an Administrative Assistant in the Science Missions Division, in charge of logistical support and arrangements for the annual peer review of Hubble proposals.
Darlene’s responsibilities for the peer review include receiving proposals, handling sensitive review materials, maintaining comprehensive databases, and setting up the meetings of the disciplinary panels and the Telescope Allocation Committee, which evaluate proposals
and make recommendations to the director of the Institute. Darlene coordinates the scrupulous process of selecting Hubble’s observations for the coming year, and ensures that it runs efficiently. This year, during one week in March, 109 astronomers met in Baltimore
for the peer review, and allocated observing time to 206 proposals out of a total of 733 submitted. The process ran smoothly, and the
successful proposers were notified of their selection within 10 days of the final review session.
During her time at the Institute, Darlene witnessed the momentous transition from paper submission of proposals to the totally electronic
version. This change brought welcome relief from the tedious tasks of filing, proofing, and mailing 30 hard copies of each proposal.
Darlene also supports the Hubble and Institute Fellowship programs. In addition to her administrative work, she arranges the meetings
of the review panels that select the fellows, and organizes the annual Hubble Symposium, where Hubble Fellows meet each other and
present their research.
Darlene is strongly involved in her community. She teaches children ages three to six, and actively participates in ministries at the
Central Church of Christ. Her hobbies consist of shopping, traveling, reading religious, inspirational, and motivational books, listening
to music, and collecting snow globes and state magnets.
Darlene resides in Owings Mills, Maryland, with one of her daughters, Dwayaa’. She spends much joyful time with her granddaughter
ZaRiah, the child of her second daughter, Unique.
127
Hubble 2006: Science Year in Review — Supporting Hubble
Pataro
Lead, Flight Operations Support Team
Lockheed Martin
Very few people who work at the Goddard Space Flight Center have been around as long as Pete Pataro. He started in 1967, as a member of the network support team for the operations center of the Apollo program. He worked on all the Apollo missions, from the early
Saturn V test flights through the final moon landing of Apollo 17. He then moved on to the Apollo-Soyuz, Skylab, and the shuttle programs before joining the next great program to come to Goddard—Hubble—in 1985.
Pete is part of the flight-operations group for Hubble. He started on the console, as a flight controller responsible for operating the data
management and communications systems of the spacecraft. Later, he became a shift supervisor, and served in that capacity for the
Hubble deployment and first servicing missions. For the second through fourth servicing missions, he again served on the console,
coordinating between the Goddard Hubble team and the payload officer at the Johnson Space Center.
Pete is now a member of the Operations Support Team under the Lockheed Martin Systems Management Office. He manages the daily
use of the operational and backup ground systems, and is the point of contact between the flight operations team and external operational
organizations, including the Space Telescope Science Institute, the Tracking Data and Relay Satellite System, the Deep Space Network,
the NASA Ground Network, and the NASA Communications and Internet Network.
Pete once said in an interview: “Working with the Hubble team on this project is the highlight of a great career. To be part of something
that makes history every day is the best place in the world to be. When people ask and I tell them what I do, they always say that my job
sounds like a lot of fun—and you know what? It is!”
Pete’s love for aeronautics began at an early age; his father was in the U.S. Air Force, and Pete grew up on Air Force bases, where the
latest planes, avionics, and electronics were constant topics.
When not supporting Hubble, Pete and his wife, Alice, enjoy visiting inspiring places to study American history, and seeking out good
restaurants to enjoy a meal and discuss what they have learned. Pete also enjoys hiking in the Blue Ridge Mountains and along the
Appalachian Trail, fly-fishing in Pennsylvania’s fabled limestone streams and creeks, and reading everything from Harry Potter books to
the history of the 8th Air Force.
128
Hubble 2006: Science Year in Review — Supporting Hubble
Plants
Resources Analyst
NASA Goddard Space Flight Center
Jean Plants was born and raised in Maryland and graduated from the University of Maryland, College Park with a Bachelor’s degree in
Economics. After graduation, she began working for the Federal Energy Regulatory Commission. Two years of commuting to Washington, DC, made Jean envious of her husband’s 15-minute commute to Goddard Space Flight Center. Along with first-hand knowledge that
Goddard is a great place to work, the easy commute made it a logical place to search for a job when she began to plan a career move. In
1987, she accepted a position as a Cost and Price Analyst in the Procurement Support Division. Four years later, Jean switched to a role
in resources and budgeting. Shortly thereafter, Jean became a Resources Analyst for the Communications Division, taking on financial
responsibilities for a $72 million budget.
Jean joined the Hubble team in 2000 as a Resources Analyst in the Operations Project. For five years, she served as the analyst for the
major contract providing for the day-to-day operations of the telescope, including the development and maintenance of its ground system. Jean prepared yearly budget estimates for the contract, monitored its monthly costs and funding needs, and supported procurement
actions—all the while keeping the contract synchronized with the project’s requirements.
Last fall, Jean became the resources analyst on the Space Telescope Science Institute contract. The contract scope includes solicitation
of observation proposals, allocation of telescope time, implementation of the weekly observing plans, and creation of processed data
products. Jean considers herself very fortunate to work for Hubble and is continually amazed by the engineering behind operating and
servicing the observatory—and by the incredible science it produces.
“I am a nature lover,” she explains. “My kids get tired of me ‘oohing’ and ‘aahing’ over the scenery we travel through—the sunsets,
clouds, stars, mountains, lakes, birds—endless natural beauty. NASA brings the beauty of the universe to us all. For me, it shows the
awesome power of God. As a non-scientist and non-engineer who could be doing budgeting work for any type of organization, I count
myself as really blessed to be able to do what I do for NASA and Hubble.”
Jean and her husband, Michael, celebrated their 20th anniversary this year and are the proud parents of Kacie, 14, and Julia, 12. Jean
and her family live in Ellicott City, Maryland, and enjoy camping, music, playing games, and visiting with their extended family.
129
Hubble 2006: Science Year in Review — Supporting Hubble
Coleman
Operations Systems Management Manager
Honeywell Technolgy Solutions, Inc.
Pat Coleman’s interest in space and astronomy started when she was young. She remembers being inspired by the children’s book she was
given, You Will Go to the Moon by Mae Blacker Freeman. It wasn’t until she watched the touchdown of the first space shuttle flight, however,
that she realized it was really time for her to get involved. It was then that Pat made a major career change, deciding to give up teaching, which
she had done for 10 years, and pursue aerospace work, which more directly utilized her education in computer programming and degree in
mathematics.
In the spring of 1984, Pat began working for the Bendix Field Engineering Corporation (now Honeywell) at the Goddard Space Flight Center.
She worked in the command management facility, providing support to two flight projects: the Solar Maximum Mission and the Solar Dynamics
Explorer. She later transferred to the Hubble mission operations program, supporting various engineering aspects of Hubble operations ever
since.
Before Hubble was launched in 1990, Pat worked in the Mission Planning Office, writing documentation and procedures to establish interfaces
between Hubble operations at Goddard with those at the Space Telescope Science Institute. She also evaluated software output products to resolve errors, and supported ground-system tests between Goddard and Lockheed in Sunnyvale, California, where the spacecraft was built. “One
of my greatest thrills was to see Hubble prior to launch,” she says. “It was impressively large, marvelously complex, and strikingly beautiful.”
By the first Hubble servicing mission, Pat was managing the project database office—building parallel-command and telemetry databases to
support operations and servicing mission development. For the next nine years, she managed a test team responsible for integrated testing of the
ground system, deliveries of flight software, and servicing-mission operations and ground-system testing of the Hubble replacement hardware
and science instruments.
Pat is a talented seamstress. She has made drapes (for 20 by 20 foot windows!) for a home listed on the historical register in Washington, DC.
Such attention to detail and the ability to oversee large projects have served her well in tackling her current Hubble assignment—leading the Systems Management Operations Team, which coordinates engineering acceptance testing, oversees on-orbit activities performed by the spacecraft
subsystem engineers, investigates anomalies, and supports operations with planning and preparations for the next servicing mission. “I consider
myself very fortunate to have been given the opportunity to continually learn and grow on the Hubble project all these years, “ she says.
130
Hubble 2006: Science Year in Review — Supporting Hubble
Walyus
Servicing Mission Operations Manager
NASA Goddard Space Flight Center
Keith Walyus joined the Hubble operations team in 1998. He has held various jobs on the project, starting as a systems engineer and
then becoming the deputy flight operations manager in 2000. He has been the Servicing Mission Operations Manager since shortly after
Servicing Mission 3B in 2002.
Keith began his NASA career in 1985 as an Air Force 2nd Lieutenant working at the Johnson Space Center in Houston, before becoming
a NASA employee in 1989. He moved to the Washington, DC area and became a Goddard Space Flight Center employee in 1993. For
Keith, working in conjunction with the staff at Johnson Space Center on a servicing mission is a kind of a homecoming. In fact, some of
the engineers and managers who are supporting the next Hubble servicing mission were fellow Air Force officers with Keith in Texas.
Keith is a member of the Goddard’s Speakers Bureau and enjoys giving talks to various internal and external organizations. “What always
impresses me is the deep and genuine interest everyone has in the status of Hubble,” he comments. “To me, it seems that Hubble has
passed beyond being just a telescope and is now a cultural icon. It gives one a tremendous feeling of accomplishment to be able to show
people pictures from Hubble, and to know that I’ve played a part in these discoveries.”
Keith and his wife, Christine, have three daughters, ages three and under. He hopes that a successful servicing mission will allow the
telescope to operate long enough for his girls to also be amazed by some of Hubble’s discoveries as they occur. In his spare time (which
isn’t too often with three small children!) Keith enjoys biking, running, and martial arts.
131
Hubble 2006: Science Year in Review — Supporting Hubble
Bean
Graduate Student
University of Texas at Austin
Jacob Bean is a fourth year doctoral candidate in astrophysics at the University of Texas in Austin. His dissertation research is focused on
determining the atmospheric compositions of the lowest-mass stars—the so-called “red dwarfs.” Building on previous work in the field,
Jacob developed the first technique to accurately determine the atmospheric compositions of these stars. He uses his newly developed
technique to quantify the relationships between composition, luminosity, and the probability of planet formation in red dwarfs.
In 2002, Jacob earned a B.S. in Physics from the Georgia Institute of Technology. After graduating, he chose to attend the University of
Texas for graduate school and to work with the Hubble Astrometry Science Team.
In addition to his dissertation research on red dwarfs, Jacob is involved in projects using Hubble’s Fine Guidance Sensors (FGSs) to
make precise measurements of the distances to planetary nebulae and Cepheid variables, the absolute dimensions of extrasolar planetary
systems, and the masses of red dwarfs. Jacob is responsible for reducing the Hubble data for these projects. He also coordinates the
collection and analysis of supporting spectroscopic data obtained at ground-based telescopes.
Jacob says he particularly enjoys working with the high-quality data from the FGSs, because Hubble was one of the reasons he chose a
career in astronomy. “The stunning images of distant galaxies, gaseous nebulae, and star clusters that Hubble produced in the late 1990s
came at a time when I was deciding what degree to pursue as an undergraduate. The popular articles that accompanied those images in
the media left me wanting to know more details.”
Outside of astronomy, Jacob enjoys competitive road cycling and long-distance swimming. He enjoys training with his wife, who shares
similar interests. Together they also enjoy hiking, traveling, cooking, and are trying to raise an unruly cat named Spock.
132
Hubble 2006: Science Year in Review — Supporting Hubble
Wechsler
Hubble Fellow
University of Chicago
Risa Wechsler is finishing her third year as a Hubble Fellow at the University of Chicago, where she works at the Kavli Institute for
Cosmological Physics. She is a theoretical cosmologist, who primarily studies the formation of large-scale structure and galaxies in
the universe.
Risa was an undergraduate in physics at MIT. She received her Ph.D. in Physics in 2001 from the University of California at Santa Cruz,
where her work focused on galaxy clustering and the assembly history of the dark matter concentrations that host galaxies. Before starting her Hubble Fellowship, she was a postdoctoral fellow for two years at the University of Michigan.
As a Hubble Fellow, Risa’s research has focused on theoretical interpretations of data from the Hubble Space Telescope and groundbased surveys relating to the evolution of concentrations of dark matter and the galaxies that form within these concentrations. Particularly important are new observations from Hubble on the clustering properties of distant galaxies and the evolution of the size, color,
and brightness of galaxies. She hopes to discover how these properties depend on the nature of the dark matter, the cosmological
parameters, and the heating and cooling of the interstellar gas available to form new stars. By varying those parameters in her models,
she can form different hypothetical universes to compare with the observations. Through such comparisons, she tests and improves the
basic assumptions of the models, leading to an improved understanding of galaxy formation and the connection between light and the
nonluminous mass in the universe.
Risa enjoys sharing the excitement of cosmology and astronomy with the public. In 2005, she gave the Compton Lectures, a series of public lectures at the University of Chicago extending over an academic quarter. Her series was entitled “The Story of Galaxy Formation in our
Universe.” This “story” has become much easier to tell with the beautiful and inspiring images taken by Hubble in the past several years.
In fall 2006, Risa became an Assistant Professor of Physics at Stanford University and at the Stanford Linear Accelerator Center, where
she is associated with the Kavli Institute for Particle Astrophysics and Cosmology. Although she enjoyed Chicago immensely, she has
been looking forward to living in San Francisco and to getting back to her native West Coast and some topographical contrast. When
she’s not working, she enjoys traveling, hiking, dancing, yoga, and eating great food.
133
Hubble 2006: Science Year in Review — Supporting Hubble
Hubble 2006: Science Year in Review
Coming Attractions
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Planets and Disks Around Nearby Stars
Since 1995, more than 200 planets have been discovered orbiting around nearby stars. The field has grown from a few
intrepid astronomers looking for the minute changes in the velocity of stars due to the perturbations of orbiting planets, to
hundreds of astronomers finding planets using radically different techniques. Kailash Sahu’s article earlier in this volume
describes how Hubble has been used to detect planets when they pass in front of the stars about which they are orbiting.
If the star is bright enough, this transit technique can be used to constrain the size of the planet and probe its atmosphere.
Hubble has successfully done this for a planet orbiting around the star HD 209458, and over the coming year will be carrying
out similar types of observations on several recently discovered transiting planets. Particularly interesting is an upcoming
attempt to detect clouds and water using infrared observations.
Another technique for finding or confirming the existence of planets is to search for tiny changes in the positions of
stars in response to the orbiting planets. To date, this “reflex motion” has been detected in just a few planetary systems.
Ongoing observations with Hubble’s Fine Guidance Sensors are aimed at detecting this motion in another half-dozen planetary systems. By combining the Hubble positional data with the velocity data from ground-based telescopes, astronomers
expect to make much more accurate estimates of the mass of the orbiting planets, and perhaps find additional planets in
those planetary systems.
A related burgeoning field is the study of dust disks around nearby stars. Such disks now appear to be common around
young stars, and are likely the ancestors of planetary systems. More and more disks are being discovered, especially because
of observations by the Spitzer infrared observatory, which can detect their infrared thermal signature. Observations by Hubble
over next few years will help to reveal the detailed structure of such disks, characterizing how their appearance changes
around stars of differing ages, and perhaps revealing how they are sculpted by the gravitational influence of unseen planets.
Page 134–135: One of the universe’s most stately and photogenic galaxies is the Sombrero galaxy, Messier 104 (M104). The galaxy’s
hallmark is a brilliant white, bulbous core encircled by the thick dust lanes which comprise its spiral structure.
Left: The parent star (highlighted with arrow) of the transiting planet mentioned in the opening paragraph on this page is called HD 209458.
It lies 150 light-years from Earth, and can be found with binoculars in the constellation of Pegasus.
137
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Nearby Clusters of Galaxies
Galaxies are not uniformly distributed in space. Instead, they tend to be found in groups and clusters, drawn together by their
mutual gravitational attraction. Interestingly, the properties of galaxies appear to be strongly dependent on their environment.
Galaxies in dense clusters tend to have older stars and are nearly devoid of the gas necessary to fuel star formation. Even
today, galaxies that are surrounded by fewer neighbors tend to have more gas and are generally forming new stars. The physical mechanisms driving these differences are only partially understood. Hubble can help uncover the answers in a variety of
ways. For example, the high resolution afforded by Hubble observations allows astronomers to find thousands of individual
star clusters within the galaxies, and differentiate them from foreground stars in our own galaxy. The colors of these star
clusters provide information on their ages and chemical abundances. By comparing the color distribution in different galaxies, astronomers hope to piece together a better history of star formation in the galaxies that inhabit nearby groups.
Hubble observations also provide detailed information on the central regions of galaxies, where stars often orbit
under the influence of a supermassive black hole. By studying large collections of galaxies, astronomers hope to better understand the connection between the black holes and the surrounding stellar populations. Beyond the central regions, detailed
measurements of galaxy shapes and color variations can help to provide information on how the galaxies have merged and
+
interacted over time. Hubble observations will also likely reveal hitherto unknown dwarf galaxies, and help to constrain how
galaxy properties vary as a function of mass. Scientific papers from the Hubble survey of the Virgo Cluster of galaxies are
now appearing in the literature, and more are expected from the major surveys of the nearby Fornax and Coma clusters that
are underway.
+
=
This is a new composite image of galaxy cluster MS0735.6+7421, located about 2.6 billion light-years away in the constellation Camelopardalis. The three views of the region were taken with NASA’s Chandra X-ray Observatory (top image on left), Hubble (second one down)
and the National Radio Astronomy Observatory Very Large Array (third one down).
139
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Stellar Populations
in Nearby Galaxies
For galaxies closer to the Milky Way than those in the nearest clusters, Hubble observations can reveal individual stars. While the
inner regions of most nearby galaxies are too crowded for a careful
census, Hubble measurements of the positions, colors, and brightness of stars in the outskirts of galaxies provides a valuable probe
of the “fossil record” of galaxy formation. Astronomers assemble
the measurements of collections of stars into diagrams of color
versus brightness, which can be compared to theoretical models
to deduce the distribution of ages and chemical compositions of
the stellar population. Over the past several years, observations
have revealed that the outskirts of galaxies contain not only ancient
stars left over from the initial phases of galaxy formation, but also
stars more recently acquired from the destruction of dwarf galaxies
by the tidal forces experienced when their orbits take them close
to much larger galaxies. Hubble observations will help determine
when, and how often, such encounters occurred.
Swirls of gas and dust reside in this ethereal-looking region of star formation imaged by Hubble. This majestic view of LH 95, located in the
Large Magellanic Cloud, reveals a region where low-mass, infant stars and their much more massive stellar neighbors reside. The image
was taken in March 2006 with Hubble’s Advanced Camera for Surveys.
141
Hubble 2006: Science Year in Review
Hubble 2006: Science Year in Review
Jupiter and Saturn in the Heliophysical Year
One of the most beautiful phenomena in the night sky is the aurora borealis,
or northern lights. These spectacular light shows occur when charged particles in the solar wind collide with atoms in Earth’s upper atmosphere. The
aurora is shaped by Earth’s magnetic field, which can cause the auroral glow
to take the form of curtains, arcs, rings, or rays.
Jupiter’s Northern Aurora
Like Earth, Jupiter and Saturn both have aurorae. An extensive Hubble observing campaign in 2007 is targeted at determining the physical relationship of
the various auroral processes at Jupiter and Saturn with conditions in the solar
wind at each planet.
The year 2007 has been designated the International Heliophysical Year,
and represents a unique period of especially concentrated measurements of
Jupiter’s Southern Aurora
space physics phenomena throughout the Solar System. The Hubble observations will be done in concert with measurements of the plasma density of
the solar wind from the New Horizons spacecraft, now on its way to Pluto,
and from the Cassini spacecraft, now in orbit around Saturn. The observations will allow astronomers to correlate the auroral
behavior with the geometry of the local magnetic field and properties of the solar wind. For Jupiter, the observations will
allow detailed study of the auroral “footprint” left by a river of electric current (of about 1 million amperes) that flows between
the planet and its volcanic moon Io.
The dancing auroras on Saturn do not behave as scientists previously predicted. New research by a team of astronomers has overturned
theories about how Saturn’s magnetic field acts and how the planet’s auroras are generated. The phenomenon is fundamentally different
from that observed on Earth or Jupiter.
143
Hubble 2006: Science Year in Review
Further Reading
Barger, A., “The Midlife Crisis of the Cosmos,” Scientific American, January 2005.
Begelman, M. C. “Evidence for Black Holes,” Science, vol. 300, 1898–1903, 20 June 2003.
DeYoung, D. S., The Physics of Extragalactic Radio Sources, University of Chicago Press, ISBN 0226144151, 2002.
Disney, M., “A New Look at Quasars,” Scientific American, June 1998.
Harris, D. E., and H. Krawczynski, “X-ray Emission from Extragalactic Jets,” Annual Reviews of Astronomy & Astrophysics, vol. 44,
463–509, 2007.
Harris, D. E., et al., “The Outburst of HST-1 in the M87 Jet,” Astrophysical Journal, 640, 211, 2006.
Hasinger, G., and R. Gilli, “The Cosmic Reality Check,” Scientific American, March 2002.
Jester, S., et al., “New Chandra Observations of the Jet in 3C 273: I. Softer X-ray than Radio Spectra and the X-ray Emission
Mechanism,” Astrophysical Journal, Vol. 648, pages 900-909, September 2006.
Johnson, G., Miss Leavitt’s Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe, W. W. Norton, New
York, 162, 2005.
Meier, D. L., S. Koide, and Y. Uchida, “Magnetohydrodynamic Production of Relativistic Jets,” Science, vol. 291, 84–92, January 2001.
Perlman, E. S., and A. S. Wilson, “The X-ray Emission from the M87 Jet: Diagnostics and Physical Interpretation,” Astrophysical
Journal, 627, 140, 2005.
Readings on Spectroscopic Selection: Bolton et al. 2004, Astronomical Journal, 127, 1860; Warren et al. 1996, Monthly Notices of the
Royal Astronomical Society, 278, 139.
Readings on SLACS Survey: Bolton et al. 2006, Astrophysical Journal, 638, 703; Treu et al. 2006, Astrophysical Journal, 640, 662;
Koopmans et al. 2006, Astrophysical Journal, 649, 599.
“Sky & Telescope,” http://skytonight.com/news/3307111.html
Turner, E. L., “Gravitational Lenses,” Scientific American, 54, July 1988.
Wambsganss, J., “Gravity’s kaleidoscope,” Scientific American, no. 5, 52–59, http://adsabs.harvard.edu/abs/2001SciAm.285e..52W, 2001.
Weaver, K., “The Galactic Odd Couple,” Scientific American, July 2003.
Uchiyama, Y., et al., “Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope,” Astrophysical Journal, 648, 910, 2006.
York et al., “Sloan Survey,” Astronomical Journal, 120, 1579, 2000.
144
Hubble 2006: Science Year in Review
Acknowledgments
Credit for the success of the Hubble Space Telescope rightly belongs to an entire universe of people and organizations. First and foremost are the citizens of the United States and Europe, who have steadfastly supported Hubble over the years with their tax dollars and
their enthusiasm. As a result, thousands of astronomers from around the world have successfully used Hubble to probe the deepest
mysteries of the universe and have shared their discoveries through both professional publications and public outreach. Educators and
students worldwide have recognized in Hubble an important source of knowledge, excitement, and motivation about science.
A small cadre of astronauts from NASA and ESA have taken significant personal risk to service Hubble, maintaining and upgrading
the spacecraft to keep it at the forefront of astronomical research. The Science Mission Directorate at NASA Headquarters and the HST
Program Office at NASA’s Goddard Space Flight Center have led the Hubble program over the years, with major contributions to the
observatory—both hardware and people—also provided by the ESA.
Hubble’s highly successful science program has been organized and guided by the Space Telescope Science Institute, operated by the
Association of Universities for Research in Astronomy under contract to NASA. Last, but not least, many dedicated NASA employees and
dozens of first-class contractor organizations throughout the global aerospace industry have designed, built, and successfully operated
Hubble and its scientific instruments over a period spanning decades.
All these people and organizations should take pride in the scientific achievements described in this publication.
For additional information, contact:
Susan Hendrix
NASA’s Goddard Space Flight Center
Office of Public Affairs
Greenbelt, MD 20771
301-286-7745
Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD 21218-2410
410-338-4444 (general info)
410-338-4707 (technical info)
http://hubblesite.org/
http://hubble.nasa.gov/
The team at Space Telescope Science Institute for this publication included Robert Brown (Editor), Henry Ferguson,
Ann Feild, Christian Lallo, Mario Livio, Sharon Toolan, and Ray Villard. The team at Goddard Space Flight Center included
Kevin Hartnett (Lead), James Jeletic, David Leckrone, Michael Marosy, Steven Stuart, Edward Henderson, Chris Gunn,
Pat Izzo, Elaine Firestone, Carol Ladd, and Mindy Deyarmin.
In reference lists, please cite this document as Hubble 2006: Science Year in Review, Robert Brown, ed. (Baltimore: STScI for
NASA Goddard Space Flight Center).
145
Hubble 2006: Science Year in Review