EMBARGOED FOR The rate of star birth during the first half of the universe's history remained high and steady prior to a sudden drop-off about 7 billion years ago, a new study shows.
Astronomers today referred to the decline in activity as the end of a cosmic baby boom.
During most of that early era, galaxies grew at a steady pace fed by frequent mergers. All the while, the initially cramped universe expanded and thinned out as available stellar raw material -- mostly hydrogen -- was drawn into galaxies and vast clumps of them called galaxy clusters.
The results, presented today, are no big surprise. But they do represent one new component of an increasingly thorough inventory of the early universe. The findings are part of an emerging deep-sky survey conducted with the Hubble Space Telescope and the Chandra X-ray Observatory.
Astronomers said the findings, which they termed preliminary, provide further evidence that major galaxy building slowed down when the universe was about half its present age of 13.7 billion years.
"This is the first time that the cosmic tale of how galaxies build themselves has been traced reliably to such early times in the universe's life," said Mauro Giavalisco, an astronomer at the Space Telescope Science Institute who led the Hubble observations.
Hubble and Chandra are in the process of examining a swath of sky containing tens of thousands of galaxies stretching back to when the universe was less than 2 billion years old. The project is called the Great Observatories Origins Deep Survey (GOODS).
Galaxy sizes increased continuously from when the universe was about 1 billion years old to when it was 6 billion years old, the study shows.
That finding is consistent with the leading theory of galaxy formation, a hierarchical model in which mergers of equals play a significant role and larger galaxies consume smaller, satellite galaxies. It also supports the idea that mysterious
dark matter, so-far invisible but known to permeate the universe, clumped early on and encouraged -- via its gravitational effects -- the development of larger and larger galaxies.Another
recent survey, of microwave radiation leftover from the young universe, suggested that the first stars were born about 200 million years after the Big Bang. But no conventional telescope has seen anything during the universe's first billion years, and researchers have not yet concluded exactly how galaxies and stars first came together.In the new work, astronomers learned that the rate of star formation, thought to be intimately related to galaxy formation and galactic mergers, tripled in the first 500 million years of the observable era. It leveled off and stayed high until about 7 billion years ago, then fell dramatically, to one-tenth its former pace.
The results will be detailed in the Astrophysical Journal Letters.
The new Chandra observations were billed as the deepest X-ray pictures ever. Interestingly, they revealed several new X-ray sources that are not related to any known optical sources. X-rays come from many objects and phenomena; strong X-ray point sources are often thought to be emitted from the
chaotic environments around black holes."We found seven mysterious sources that are completely invisible in the optical with Hubble," said Niel Brandt of Penn State University, who presented the Chandra discoveries. "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."
More research is needed to figure out what has been found.
If all goes well, the GOODS program will soon gain contributions from another space-based telescope, tentatively named
SIRTF. This infrared telescope was slated to launch this spring but delays have pushed liftoff into to August.More Deep Space News 
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