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Donald Savage
Headquarters, Washington
(Phone: 202/358-1547)           June 19, 2003

Nancy Neal
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-0039)

Ray Villard
Space Telescope Science Institute, Baltimore
(Phone: 410/338-4514)

Megan Watzke
Chandra X-ray Observatory Center, CfA, Cambridge, Mass.
(Phone: 617/496-7998)

RELEASE: 03-199

THE SECRET LIVES OF GALAXIES UNVEILED IN DEEP SURVEY

     Two of NASA's Great Observatories, bolstered by the 
largest ground-based telescopes around the world, are 
beginning to harvest new clues to the origin and evolution 
of galaxies. It's a bit like finding a family scrapbook 
containing snapshots that capture the lives of family 
members from infancy through adolescence to adulthood.

"This is the first time the cosmic tale of how galaxies 
build themselves has been traced reliably to such early 
times in the universe's life," said Mauro Giavalisco, head 
of the Hubble Space Telescope (HST) portion of the survey, 
and research astronomer at the Space Telescope Science 
Institute (STScI) in Baltimore.

The HST has joined forces with the Chandra X-ray Observatory 
to survey a relatively broad swath of sky encompassing tens 
of thousands of galaxies stretching far back into time. The 
Space Infrared Telescope Facility (SIRTF), scheduled for 
launch in August, will soon join this unprecedented survey. 
Called the Great Observatories Origins Deep Survey (GOODS), 
astronomers are studying galaxy formation and evolution over 
a wide range of distances and ages. The project is tracing 
the assembly history of galaxies, the evolution of their 
stellar populations, the gusher of energy from star 
formation and active nuclei powered by immense black holes.

HST astronomers report the sizes of galaxies clearly 
increase continuously from the time the universe was about 1 
billion years old to an age of 6 billion years; 
approximately half the current age of the universe, 13.7 
billion years. GOODS astronomers also find star birth rate 
rose mildly, by about a factor of three, between the time 
the universe was about one billion years old and 1.5 billion 
years old. It remained high until about 7 billion years ago, 
when it quickly dropped to one-tenth the earlier "baby 
boomer" rate. This is further evidence major galaxy building 
trailed off when the universe was about half its current 
age.

This increase in galaxy size is consistent with "bottom-up" 
models, where galaxies grow hierarchically, through mergers 
and accretion of smaller satellite galaxies. This is also 
consistent with the idea the sizes of galaxies match hand-
in-glove to a certain fraction of the sizes of their dark-
matter halos. Dark matter is an invisible form of mass that 
comprises most of the matter in the universe. The theory is 
dark matter essentially pooled into gravitational "puddles" 
in the early universe, then collected normal gas that 
quickly contracted to build star clusters and small 
galaxies. These dwarf galaxies merged piece-by-piece over 
billions of years to build the immense spiral and elliptical 
galaxies we see today.

The Chandra observations amounted to a "high-energy core 
sample" of the early universe, allowing us to "study the 
history of black holes over almost the entire age of the 
universe," said Niel Brandt of Penn State University, a co-
investigator on the Chandra GOODS team. One of the 
fascinating findings in this deepest X-ray image ever taken 
is the discovery of mysterious black holes, which have no 
optical counterparts.

"We found seven mysterious sources that are completely 
invisible in the optical with Hubble," said Anton Koekemoer 
of the STScI, a co-investigator on both the HST and Chandra 
GOODS teams. "Either they are the most distant black holes 
ever detected, or they are less distant black holes that are 
the most dust enshrouded known, a surprising result as 
well."

When comparing the HST and Chandra fields, astronomers also 
found active black holes in distant, relatively small 
galaxies were rarer than expected. This may be due to the 
effects of early generations of massive stars that exploded 
as supernovae, evacuating galactic gas and thus reducing the 
supply of gas needed to feed a super massive black hole.

These and other results from the GOODS project will be 
published in a special issue of the Astrophysical Journal 
Letters, entirely devoted to the team's results. The Chandra 
results are found in papers led by Koekemoer and Stefano 
Cristiani of the Trieste Astronomical Observatory. Hubble's 
findings came from papers led by Giavalisco, Mark Dickinson, 
and Harry Ferguson of the STScI.

Electronic images and additional information are available 
on the Internet at:
http://hubblesite.org/news/2003/18
http://chandra.harvard.edu

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