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National Aeronautics and Space Administration
Fossil Galaxies
Taken from:
Hubble 2012: Science Year in Review
TakenProduced
from: by NASA Goddard Space Flight Center
and the Space Telescope Science Institute.
Hubble
2012: Science Year in Review
The full contents of this book include Hubble science articles, an overview of
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NASA Goddard
and more.
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online
Institute.
at:
www.hubblesite.org/hubble_discoveries/science_year_in_review
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
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HUBBLE 2012: SCIENCE YEAR IN REVIEW
Ultra-Faint Fossil Galaxies
Astronomers and cosmologists have a problem on their hands. Their trusted computer simulations of the evolving universe
predict that thousands of dwarf galaxies should currently exist in orbit around our Milky Way galaxy. However, only a few
dozen dwarf galaxies have been observed. Called the “missing satellite problem,” one possible explanation is that there has
been very little or, perhaps, no star formation in the smallest of these dwarf galaxies, making them difficult to detect.
During the past decade, astronomers using automated computer techniques to search through the images of the groundbased Sloan Digital Sky Survey have indeed discovered a new class of star-starved, ultra-faint dwarf galaxies. They are the
least luminous, most dark-matter-dominated, and least chemically evolved galaxies known. These systems are thought to
be some of the tiniest, oldest, and most pristine galaxies in the universe. The Sloan survey uncovered more than a dozen of
these galaxies in the Milky Way’s neighborhood while scanning just a quarter of the celestial sphere.
When these galaxies were discovered, astronomers proposed many reasons for their shortage of stars. Some believed
that internal dynamics, such as supernova blasts, blew out the gas needed to create more stars. Others suggested that the
galaxies simply used up what little gas they had. A few thought that these galaxies were born during the early universe and
that the harsh environment in which they were formed disrupted their ongoing star creation.
Maryland-based astronomer Thomas Brown recently used Hubble to peer deep inside six of these galaxies to examine their
stellar populations and determine their ages. The first three of these systems were Hercules, Leo IV, and Ursa Major I. The
galaxies range in distance from 330,000 to 490,000 light-years from Earth.
Brown and his team precisely measured the stars’ ages by analyzing their brightness and colors with Hubble. For reference,
Brown compared the galaxies’ stars with the stars in the ancient, globular cluster M92, located 26,000 light-years away.
This Hubble view reveals the heart of Leo IV, one of more than a dozen ultra-faint dwarf galaxies located around the Milky Way. The field
of view is approximately 480 light-years wide. Leo IV resides 500,000 light-years from Earth and has so few stars—roughly several
thousand—that astronomers had difficulty identifying it as a galaxy.
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HST /ACS
Sloan Digital Sky Survey
The large background field shows the region in the Sloan Digital Sky Survey where the ultra-faint dwarf galaxy Leo IV resides. Scientists discovered the
galaxy by noticing a region where a collection of stars was grouped closer together than stars in the areas around it. The irregular outline in the image
marks the estimated perimeter of Leo IV, while the pop-out Hubble image shows a detail of the galaxy.
M92 is more than 13 billion years old, one of the oldest objects in the universe. The team’s analysis revealed that the three
galaxies’ stars are all the same age and very similar to those in M92.
These galaxies evidently started forming stars more than 13 billion years ago—and then abruptly stopped within the span of
about a billion years. What could shut down star formation at the same time in all three galaxies? Researchers suggest that
a universal event, like the reionization of the intergalactic medium could have been the cause.
The period called reionization is a transitional phase thought to have occurred in the early universe when the very first
stars and galaxies “burned off” a residual fog of cold, intergalactic hydrogen. During this epoch, radiation from the first
stars knocked electrons off primeval hydrogen atoms, ionizing the hydrogen gas. This process allowed the hydrogen gas
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to become transparent to ultraviolet light so that stars could be seen. Ironically, the same radiation that sparked universal
reionization and made stars visible across the universe also appears to have stopped star-making activities in dwarf galaxies,
such as those in Brown’s study. These small, irregular galaxies were born about 100 million years before reionization began
and had just started to create stars.
Scientists believe that these diminutive systems—each about 2,000 light-years across—did not possess sufficient interstellar
gas to shield themselves from the harsh ultraviolet light radiating from their own massive stars or from the light of similar
stars in nearby galaxies. What little gas they had was stripped away, and the galaxies lost the raw material to create additional
stars. As a result, these galaxies are “living fossils” formed during a bygone age that have not changed in billions of years.
The image at left shows part of the ultra-faint dwarf galaxy Leo IV, outlined by the white rectangular box. The box represents 83 light-years in width by
163 light-years in height. The few stars in Leo IV are lost amid neighboring stars and distant galaxies. A close-up view of the background galaxies within
the box appears in the middle image. The image at right shows only the stars in Leo IV within the same field. The tiny galaxy, which contains several
thousand stars, is composed of Sun-like stars, fainter red dwarf stars, and some red giant stars brighter than the Sun.
HST /ACS
Background galaxies
Leo IV stars
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Simulation of dark matter in the Milky Way halo
These computer simulations of the
evolving universe predict dark-matter
concentrations around the Milky Way
galaxy, which appears in the center of these
images. Some are massive enough to host
star formation. The first panel shows the
thousands of dark-matter clumps that
coexist with our galaxy. The second panel
highlights in green those dark-matter
clumps massive enough to obtain gas
from the intergalactic medium and trigger
star formation, eventually creating dwarf
galaxies. The third panel highlights in
red those dark-matter clumps where star
formation began but abruptly stopped—
these are the ultra-faint dwarf galaxies. The
simultaneous termination of star formation
about 13 billion years ago is evidence that
a global event, such as reionization, swept
through the early universe.
These galaxies are unlike most
nearby
galaxies,
which
have
extended star-formation histories.
The stellar populations in these
Dwarf galaxies with ongoing star formation
relic galaxies range from a few
hundred to a few thousand stars,
both fainter and brighter than the
Sun, but all born at the same time.
The galaxies may be star-deprived,
but analysis of the motions of their
stars reveals these galaxies have
an abundance of dark matter, an
invisible substance that makes up
the bulk of the universe’s mass and
Dwarf galaxies with star formation shut off by reionization
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the underlying scaffolding upon
which galaxies are built.
Normal dwarf galaxies near the Milky Way contain 10 times more dark matter than the ordinary matter that makes up gas
and stars. In ultra-faint dwarf galaxies, dark matter outweighs ordinary matter by at least a factor of 100. Because their gas
was expelled and their star formation quenched, the small galaxies in Brown’s study are made up of mostly dark matter.
Thus, these dark-matter islands where dwarf galaxies formed coexisted unseen with our Milky Way for billions of years,
until astronomers began finding them in the Sloan survey. The evidence for the abruptly halted star formation in some of
the smallest of these dwarfs suggests that there may be thousands more that formed even fewer stars. If so, the predicted
population of satellite galaxies is not missing after all, but rather just difficult to find.
Further Reading
Belokurov, V., et al. “Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions.” The Astrophysical Journal 654
(January 10, 2007): 897–906.
Brown, T. M., et al. “The Primeval Populations of the Ultra-Faint Dwarf Galaxies.” The Astrophysical Journal Letters 753
(July 1, 2012): L21, doi: 10.1088/2041–8205/753/1/L21.
Cartlidge, E. “What Reionized the Universe?” Science 336 (June 1, 2012): 1095–1096.
McConnachie, A. W., et al. “The Remnants of Galaxy Formation From a Panoramic Survey of the Region Around M31.” Nature 461
(September 3, 2009): 66–69, doi:10.1038/nature08327.
Williams, J. “Hubble Spies Tiny, Ancient ‘Ghost Galaxies.’”
http://www.universetoday.com/96230/hubble-spies-tiny-ancient-ghost-galaxies/.
Zucker, D. B., et al. “Andromeda X, a New Dwarf Spheroidal Satellite of M31: Photometry.” The Astrophysical Journal 659
(April 10, 2007): L21–L24.
Dr. Thomas M. Brown grew up on Long Island, New York, in the town of Commack. He obtained a bachelor of
science from the Pennsylvania State University, with a double major in physics and astronomy/astrophysics.
In 1992, he moved to Baltimore, where he attended The Johns Hopkins University. While there, he worked on
the Hopkins Ultraviolet Telescope and its associated space shuttle mission. After earning his master of arts in
1994 and his doctorate in 1996 from JHU, he worked at NASA’s Goddard Space Flight Center on the Instrument
Definition Team for Hubble ’s Space Telescope Imaging Spectrograph. In 2001, he became an Instrument
Scientist for the Space Telescope Science Institute. After supporting Hubble for seven years, he became the
mission scientist for the James Webb Space Telescope .
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