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National Aeronautics and Space Administration 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 Producedthe bytelescope, NASA Goddard and more. SpaceThe Flight complete Centervolume and its component sections are and the Space available Telescope for download Science 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 76 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. 77 HUBBLE 2012: SCIENCE YEAR IN REVIEW 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 78 HUBBLE 2012: SCIENCE YEAR IN REVIEW 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 79 HUBBLE 2012: SCIENCE YEAR IN REVIEW 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 80 HUBBLE 2012: SCIENCE YEAR IN REVIEW 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 . 81 HUBBLE 2012: SCIENCE YEAR IN REVIEW