Donald Savage<br />Headquarters, Washington<br />(Phone: 202/358-1547) June 19, 2003<br /><br /><br />Nancy Neal<br />Goddard Space Flight Center, Greenbelt, Md.<br />(Phone: 301/286-0039)<br /><br /><br />Ray Villard<br />Space Telescope Science Institute, Baltimore<br />(Phone: 410/338-4514)<br /><br /><br />Megan Watzke<br />Chandra X-ray Observatory Center, CfA, Cambridge, Mass.<br />(Phone: 617/496-7998)<br /><br /><br />RELEASE: 03-199<br /><br /><br />THE SECRET LIVES OF GALAXIES UNVEILED IN DEEP SURVEY<br /><br /><br /> Two of NASA's Great Observatories, bolstered by the <br />largest ground-based telescopes around the world, are <br />beginning to harvest new clues to the origin and evolution <br />of galaxies. It's a bit like finding a family scrapbook <br />containing snapshots that capture the lives of family <br />members from infancy through adolescence to adulthood.<br /><br /><br />"This is the first time the cosmic tale of how galaxies <br />build themselves has been traced reliably to such early <br />times in the universe's life," said Mauro Giavalisco, head <br />of the Hubble Space Telescope (HST) portion of the survey, <br />and research astronomer at the Space Telescope Science <br />Institute (STScI) in Baltimore.<br /><br /><br />The HST has joined forces with the Chandra X-ray Observatory <br />to survey a relatively broad swath of sky encompassing tens <br />of thousands of galaxies stretching far back into time. The <br />Space Infrared Telescope Facility (SIRTF), scheduled for <br />launch in August, will soon join this unprecedented survey. <br />Called the Great Observatories Origins Deep Survey (GOODS), <br />astronomers are studying galaxy formation and evolution over <br />a wide range of distances and ages. The project is tracing <br />the assembly history of galaxies, the evolution of their <br />stellar populations, the gusher of energy from star <br />formation and active nuclei powered by immense black holes.<br /><br /><br />HST astronomers report the sizes of galaxies clearly <br />increase continuously from the time the universe was about 1 <br />billion years old to an age of 6 billion years; <br />approximately half the current age of the universe, 13.7 <br />billion years. GOODS astronomers also find star birth rate <br />rose mildly, by about a factor of three, between the time <br />the universe was about one billion years old and 1.5 billion <br />years old. It remained high until about 7 billion years ago, <br />when it quickly dropped to one-tenth the earlier "baby <br />boomer" rate. This is further evidence major galaxy building <br />trailed off when the universe was about half its current <br />age.<br /><br /><br />This increase in galaxy size is consistent with "bottom-up" <br />models, where galaxies grow hierarchically, through mergers <br />and accretion of smaller satellite galaxies. This is also <br />consistent with the idea the sizes of galaxies match hand-<br />in-glove to a certain fraction of the sizes of their dark-<br />matter halos. Dark matter is an invisible form of mass that <br />comprises most of the matter in the universe. The theory is <br />dark matter essentially pooled into gravitational "puddles" <br />in the early universe, then collected normal gas that <br />quickly contracted to build star clusters and small <br />galaxies. These dwarf galaxies merged piece-by-piece over <br />billions of years to build the immense spiral and elliptical <br />galaxies we see today.<br /><br /><br />The Chandra observations amounted to a "high-energy core <br />sample" of the early universe, allowing us to "study the <br />history of black holes over almost the entire age of the <br />universe," said Niel Brandt of Penn State University, a co-<br />investigator on the Chandra GOODS team. One of the <br />fascinating findings in this deepest X-ray image ever taken <br />is the discovery of mysterious black holes, which have no <br />optical counterparts.<br /><br /><br />"We found seven mysterious sources that are completely <br />invisible in the optical with Hubble," said Anton Koekemoer <br />of the STScI, a co-investigator on both the HST and Chandra <br />GOODS teams. "Either they are the most distant black holes <br />ever detected, or they are less distant black holes that are <br />the most dust enshrouded known, a surprising result as <br />well."<br /><br /><br />When comparing the HST and Chandra fields, astronomers also <br />found active black holes in distant, relatively small <br />galaxies were rarer than expected. This may be due to the <br />effects of early generations of massive stars that exploded <br />as supernovae, evacuating galactic gas and thus reducing the <br />supply of gas needed to feed a super massive black hole.<br /><br /><br />These and other results from the GOODS project will be <br />published in a special issue of the Astrophysical Journal <br />Letters, entirely devoted to the team's results. The Chandra <br />results are found in papers led by Koekemoer and Stefano <br />Cristiani of the Trieste Astronomical Observatory. Hubble's <br />findings came from papers led by Giavalisco, Mark Dickinson, <br />and Harry Ferguson of the STScI.<br /><br /><br />Electronic images and additional information are available <br />on the Internet at:<br />http://hubblesite.org/news/2003/18<br />http://chandra.harvard.edu