Download The Search for the Earliest Galaxies

National Aeronautics and Space Administration
The Search for the
Earliest Galaxies
Taken from:
Hubble 2010: Science Year in Review
Produced by NASA Goddard Space Flight Center
and the Space Telescope Science Institute.
The full contents of this book include Hubble science articles, an overview of
the telescope, and more. The complete volume and its component sections are
available for download online at:
www.hubblesite.org/hubble_discoveries/science_year_in_review
HUBBLE 2010: SCIENCE YEAR IN REVIEW
The Search for the Earliest Galaxies
Just twenty years ago, astronomers could only see galaxies in any detail out to a distance of approximately 7 billion lightyears—about halfway across the known universe. Thanks to Hubble and other modern observatories, this situation has
changed dramatically in the past two decades. Finding fainter and more distant galaxies dating back to the time when they
are thought to have first formed, however, continues to be a priority for modern cosmology. Astronomers are still looking to
identify the steps that led from the formation of the first stars to the immense collections of stars seen in magnificent galaxies
like our Milky Way.
In their early views with Hubble, astronomers realized that the telescope’s location above Earth’s atmosphere allowed it to
see much more distant galaxies than ever before, though these galaxies were very faint and small. Light from the galaxies
was detected in deeper and deeper shades of red as these increasingly faint objects were found at increasing distances. This
effect is due to the expansion of the universe and is called cosmological redshift. (See sidebar on page 123.)
To continue the pursuit of the very most distant galaxies, Hubble required a detector that was sensitive not only to deep
red, but also near-infrared wavelengths of light. In 1997, during Servicing Mission 2, astronauts added the Near Infrared
Camera and Multi-Object Spectrometer (NICMOS) to the telescope’s instrument complement. Its detectors significantly
extended Hubble’s “reach,” but the technology available at the time that NICMOS was built limited its inherent field of view
and sensitivity.
The near-infrared imaging capabilities of the telescope increased significantly with the installation of the Wide Field Camera 3
(WFC3) during the final servicing mission to Hubble in 2009. WFC3 enlarged the field of view by a factor of six, provided a
higher near-infrared sensitivity, and, with an increased number of pixels, improved the sharpness of the images by a factor
of three over NICMOS’s wide-area camera.
Taken by Hubble in near-infrared light, this image shows the most distant galaxies ever detected. The faintest objects are about one-billionth
as bright as can be seen with the naked eye. Since near-infrared light is invisible to the human eye, colors were assigned to represent the
distribution of the wavelengths collected. Comparatively short near-infrared wavelengths were rendered blue, medium ones green, and the
longest ones red.
119
HUBBLE 2010: SCIENCE YEAR IN REVIEW
The Reionization Era
Based on a remarkable series of ground- and space-based
observations during the last two decades, astronomers have
outlined the Cold Dark Matter theory of cosmology, which posits
that the universe began with a Big Bang some 13.7 billion years
ago. At its inception, the universe was filled with a hot plasma of
hydrogen and helium ions as well as free-moving electrons. As the
universe expanded and the plasma cooled, the electrons slowed
down sufficiently to be captured by the hydrogen and helium nuclei,
resulting in the formation of neutral hydrogen and helium atoms.
This phase transition is known as Recombination and is calculated
to have occurred when the universe was approximately 370,000 years
old. Prior to Recombination, the universe was impenetrable by light
because photons could not travel far before being scattered by the
abundant free electrons. After Recombination, when most electrons
had combined with subatomic nuclei to form complete atoms, the
universe became transparent.
This transition ushered in the Dark Ages, a period when there was
no source of visible light in the universe because stars had not yet
formed. This era may have lasted for a few hundred million years.
Gradually, the gravitational collapse of dense regions triggered the
birth of the first stars and galaxies. This star-forming period probably
lasted for 100 million to 200 million years, starting about 200
million years after the Big Bang. Eventually, the cold hydrogen atoms
in the low-density space between galaxies became so energized
by ultraviolet light from the first stars and galaxies that they were
re-stripped of their electrons, or reionized. This Reionization Era
was a critical watershed in the evolving universe. It was when the
intergalactic medium became substantially ionized as we see it today.
This reionization was completed approximately 1 billion years after
the Big Bang.
HUBBLE 2010: SCIENCE YEAR IN REVIEW
According to the Cold Dark Matter theory of
cosmology, the first stars and galaxies formed about
400 million years after the Big Bang. The most
distant and hence earliest galaxy seen by Hubble has
a redshift value z of approximately 10.
Reionization Era
The universe’s primordial helium experienced the same reionizing
transition, but at a different point in its history since more energy
is required to strip electrons from helium nuclei. A recent discovery
by astronomers using Hubble indicates that this era occurred from
11.7 billion to 11.3 billion years ago. (See Understanding the Helium
Reionization Era article, page 131). During this period, large galaxies
more frequently collided, which engorged the supermassive black
holes at their cores with gas. The accretion of material onto the black
holes produced quasars which then emitted powerful, far-ultraviolet
radiation that provided enough energy to reionize the helium. This
radiation also interrupted the growth of some small galaxies because
the gas clouds within them became too hot to collapse and form
stars. Only after the clouds cooled could small galaxies resume their
normal star-making activities.
With this enhanced resolution and sensitivity, WFC3 is now capturing images of the earliest and most distant galaxies
ever seen. Only a handful of galaxies had been found previously at distances corresponding to “look-back times” of more
than 12.9 billion years. In less than two years using WFC3, this observed primordial population has grown to about 140.
Because they are so far away, the newly discovered galaxies can only be seen in near-infrared light—not at all in WFC3’s
visible-alight channel. This belies the fact that the galaxies are actually radiating brightly at wavelengths that are ultra-blue.
Determining the Age of Galaxies
The redshift value z is a measure of the stretching of the wavelength or “reddening” of starlight due to the expansion of
space. The near-infrared data from WFC3 reveals galaxies at redshifts from z~7 to beyond z~8. The detection of galaxies
at these redshifts corresponds to look-back times of approximately 12.9 billion to 13.1 billion years ago, just 600 milliion
This is the most sensitive infrared image taken of the universe to date. The farthest galaxy ever seen appears as a faint red blob in this ultra-deepfield Hubble exposure. The dim object is a compact galaxy of blue stars that existed 480 million years after the Big Bang, when the universe was
only four percent of its current age. It is tiny and considered a building block of today’s giant galaxies. Based on the object’s color, astronomers
believe it is 13.2 billion light-years away.
121
HUBBLE 2010: SCIENCE YEAR IN REVIEW
to 800 million years after the Big Bang. Astronomers inferred the redshifts by comparing the galaxies’ brightnesses in
different near-infrared wavelength bands with visual-wavelength images recorded by Hubble’s Advanced Camera for Surveys
(ACS) from 2004 to 2010. They also combined the new Hubble data with observations from NASA’s Spitzer Space Telescope to estimate the ages and masses of the galaxies.
These galaxies are at the earliest stage of development ever seen. They appear clumpy, not yet having developed into the
familiar spiral or elliptical shapes seen in the nearby universe. They are also very compact, only one-twentieth the size of the
Milky Way galaxy, and possess only one-hundreth of its mass.
Once all the images are color-adjusted to compensate for redshift, these earliest galaxies appear bright blue. The intrinsic color
of these primordial galaxies is mainly due to their large population of young blue stars, which lack heavy elements. Heavy
elements are produced over time by nuclear processes within stars and dispersed into space through end-of-life supernovas.
As a result, these early galaxies are essentially free of the dust made from heavy elements that scatters and reddens light.
Garth Illingworth from the University of Spectral lines are the black lines in the spectrum that indicate the chemical elements present in a
California, Santa Cruz led the survey team
that used WFC3 to gather the near-infrared
data that revealed these distant galaxies.
They observed a small patch of sky called the
Hubble Ultra Deep Field (HUDF). Hubble’s
Advanced Camera for Surveys had imaged
this same field in visible light in 2004. WFC3
peered even deeper into the same section of
sky for the HUDF09 survey, which was carried
out in separate segments during the late
summers of both 2009 and 2010. The total
observation took approximately 130 hours of
Hubble ’s observing time. Once transmitted
to the ground, the data was immediately
put into the Hubble data archive and made
available to astronomers worldwide.
122
HUBBLE 2010: SCIENCE YEAR IN REVIEW
star. Astronomers are able to measure the Doppler shift by observing the lines’ positions relative
to those of the same elements in a laboratory on Earth.
Cosmological Redshift
All wave phenomena—whether ocean waves, sound waves, or light waves—can be described in terms of a few common
parameters. Among these are amplitude (the wave’s height), frequency (how many waves pass a point in a certain period
of time) and wavelength (the distance between two successive peaks). For waves moving at a constant speed, it is easy
to see that the shorter the wavelength, the higher the frequency.
Waves can also be stretched or compressed. The sound waves emitted from a passing siren get compressed into shorter
wavelengths as the siren approaches, causing the frequency of them (the siren’s pitch) to go up. Once the siren has
passed the opposite happens: the wavelengths get stretched out, causing the pitch to go down. This frequency shift
for a moving wave was first explained by Austrian physicist Christian Doppler in 1842, and as a result bears the name
“Doppler” shift.
When light waves are emitted from a moving object like a star, there is a similar, measurable effect. If the star is moving
toward Earth, the wavelength of its light becomes shorter, appearing bluer. Its light is said to be blueshifted. Conversely,
when the star is receding from Earth, its light is said to be redshifted. Scientists can calculate the object’s velocity by
measuring how much its light is shifted. The greater the shift, the faster the speed is toward or away from the observer.
Measuring the Doppler shift for a moving star is made easier by the
presence of lines in the star’s spectrum. The location of these lines
acts as a chemical fingerprint and indicates the atomic species present
in the star. By comparing the positions of these fingerprints with
respect to those of documented frequencies produced in a laboratory,
scientists can measure the amount of observed displacement.
Aside from a few dwarf galaxies in our Local Group, all galaxies in
the universe are redshifted. This is not a true representation of a
galaxy’s motion through space. Rather, the redshift is cause by the
expansion of space itself. Light is stretched to longer wavelengths
by the stretching of space. This is called the cosmological redshift.
Those few nearby galaxies that have a true Doppler-shift component
are said to have peculiar motion.
Redshift
As a photon moves through space, its wavelength stretches
along with the expanding universe, similar to the way the
length of a wavy line increases on an expanding balloon. This
stretching of light is called cosmological redshift, and makes
objects appear increasingly redder with distance.
The farther away a galaxy is, the greater its cosmological redshift. By
measuring its redshift, one then can estimate its distance. Typically,
however, insufficient light reaches Earth from the most distant galaxies
to obtain spectra bright enough to measure individual spectral lines.
The redshift of a galaxy instead is estimated by recording how
bright the entire galaxy appears through different colored filters. At
very great distances, these galaxies are only visible at the reddest
wavelengths of light. At even farther distances, their light is stretched
past red into the infrared portion of the electromagnetic spectrum.
Redshift is usually abbreviated as z and is calculated in a nonlinear
fashion. For example, z=0 means that there was no change in the wavelength of light since emission. Stars and galaxies
very close to Earth and the Milky Way have redshifts near zero. At z=1, objects are at a distance of approximately 7.7
billion light-years. At redshift z=2, this increases to about 10 billion light-years. The most distant galaxies have redshifts
around 9–10. Redshift z=10 implies a distance of just over 13 billion light-years.
HUBBLE 2010: SCIENCE YEAR IN REVIEW
Illingworth and his team member Rychard Bouwens announced and published the first discovery from the data set after
almost a year of careful analysis. The scientists found a dim object in the data called UDFj-39546284n that was measured to
have a redshift of z~10, a new astronomical record. This redshift implies that the object existed only 480 million years after
the Big Bang, though its formation is thought to have started 100 million to 200 million years earlier. It is an astronomically
tiny object; more than 100 of such mini-galaxies would be needed to equal our Milky Way. The object is only visible at the
longest infrared wavelengths detectable by Hubble.
Additionally, Bouwens and Illingworth have found evidence that there was a ten-fold increase in the rate of star formation
in the period between 480 million and 650 million years after the Big Bang. This magnitude of change in such a short
period of time is unanticipated and suggests that more surprises may be found in the light from the distant objects
slightly beyond Hubble’s detection.
Investigating the Reionization Era
The deep photographic exposures by WFC3 that uncovered the new galaxies were executed with roughly 40 times greater
efficiency than with Hubble’s earlier infrared camera, the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS).
WFC3 accomplished in five days of observing what would have previously taken almost half a year for NICMOS to do.
Examining this data set and others, astronomers have found that beyond a certain distance, the number of galaxies per unit
volume of space drops off smoothly with increasing distance. Thus, at a certain point in time, a progressive buildup began
where small objects merged to form larger ones through collision and agglomeration. The findings from the HUDF09 survey
support this hierarchical model of galaxy assembly.
Still, there is a big mystery that remains unsolved. Current galaxy counts in the era between 400 million and 900 million
years after the Big Bang are insufficient to account for the large amount of radiation believed to have reionized the universe
during this period. (See sidebar on the Reionization Era.) Perhaps the density of the very faintest galaxies—those beyond
Hubble’s detection limit—is so high that there may be enough of them to validate the theory. Or perhaps there was an earlier
wave of galaxy formation that dissipated and was followed by a second, smaller wave. Finally, perhaps the earliest galaxies
were extraordinarily efficient at reionizing the universe.
124
HUBBLE 2010: SCIENCE YEAR IN REVIEW
Further analysis of the HUDF09 data, as well as new infrared data from the much larger Hubble successor, the James Webb
Space Telescope, will no doubt be used to help solve the mystery.
Further Reading
Buchen, L. “Researchers Claim Most Distant Galaxies Yet.” Nature News, December 24, 2009,
http://www.nature.com/news/2009/091224/full/news.2009.1165.html
Cowen, R. “Galaxies May Be Most Distant Yet.” Science News 177, no. 3 (January 30, 2010): 5–6.
Gardner, J. P. “Finding the First Galaxies.” Sky & Telescope 119, no. 1 (January 2010): 24–30.
Hadhazy, A. “Building the Galaxies.” Discover 31, no. 4 (May 2010): 11–13.
Talcott, R. “Galaxies Near the Dawn of Time.” Astronomy 38, no. 5 (May 2010): 56–61.
www.firstgalaxies.org
Dr. Garth Illingworth is a professor of astronomy at the University of California, Santa Cruz and an astronomer
at the University of California Observatories/Lick Observatory. He received a doctorate in astrophysics from
the Australian National University in Canberra, Australia. His primary research interests are directed toward
understanding when and how galaxies formed, particularly the buildup of galaxies early in the universe. He was
deputy principal investigator on Hubble’s Advanced Camera for Surveys, and former chair of the Astronomy
and Astrophysics Advisory Committee to the U.S. Congress. Dr. Illingworth helped develop the concept for an
8-meter–diameter cooled infrared space telescope, which is now the James Webb Space Telescope. He was a
member of an independent review panel which recently evaluated Webb for NASA and Congress, and was Chair of
the European Southern Observatory Visiting Committee. Dr. Illingworth is also Chair of the James Webb Advisory
Council, which advises program managers on how to make Webb as scientifically productive as possible.
125
HUBBLE 2010: SCIENCE YEAR IN REVIEW