Astronomers trace the evolution of the first galaxies in the universe (Forwarded)

University of California-Santa Cruz

Contact:
Tim Stephens (831) 459-2495

September 13, 2006

Astronomers trace the evolution of the first galaxies in the universe

A systematic search for the first bright galaxies to form in the early
universe has revealed a dramatic jump in the number of such galaxies
around 13 billion years ago. These observations of the earliest stages in
the evolution of galaxies provide new evidence for the hierarchical theory
of galaxy formation -- the idea that large galaxies built up over time as
smaller galaxies collided and merged.

Astronomers Rychard Bouwens and Garth Illingworth at the University of
California, Santa Cruz, used the Hubble Space Telescope to explore the
formation of galaxies during the first 900 million years after the Big
Bang. They reported their latest findings in the September 14 issue of the
journal Nature.

Deep observations in three dark patches of sky -- the Hubble Ultra Deep
Field and the Great Observatories Origins Deep Survey fields -- gathered
the faint light emitted 13 billion years ago by stars in primeval
galaxies. Only the brightest galaxies could be detected at such great
distances.

"These are the deepest infrared and optical data ever taken. We're looking
at a very early stage in the buildup of galaxies," said Illingworth, a
professor of astronomy and astrophysics at UCSC.

The researchers observed hundreds of bright galaxies at around 900 million
years after the Big Bang. But when they looked deeper, about 200 million
years earlier in time, they only found one. Relaxing their search criteria
a bit turned up a few more candidates, but clearly a lot of changes took
place during those 200 million years, Illingworth said.

"The bigger, more luminous galaxies just were not in place at 700 million
years after the Big Bang. Yet 200 million years later there were many more
of them, so there must have been a lot of merging of smaller galaxies
during that time," he said.

Astronomers can determine when light was emitted from a distant source by
its redshift, a measure of how the expansion of the universe stretched the
wavelengths of the light as it traveled through space across vast
distances. Bouwens, a postdoctoral fellow at UCSC and first author of the
Nature paper, developed software to systematically sift through the Hubble
data in search of high-redshift galaxies.

The data came from two powerful instruments on Hubble, the Advanced Camera
for Surveys (ACS) and the Near Infrared Camera and Multi-Object
Spectrograph (NICMOS). The researchers compared the numbers of galaxies
detected at a redshift of 7 to 8 (700 million years after the Big Bang)
with what they might have expected to find if the population of galaxies
then were like the population they had observed at redshift 6 (200 million
years later). Depending on the strictness of their selection criteria,
they found one galaxy where they would have expected 10, or four where
they would have expected 17.

"Our approach provides a very quantitative way of measuring the buildup of
structure in the universe, so we can see how fast it changed over time as
smaller galaxies merged to form larger ones," Bouwens said.

The galaxies observed in this survey are much smaller than our own Milky
Way and other giant galaxies seen today in the nearby universe. These
early galaxies were also ablaze with star formation, emitting bluish light
that was shifted to red light during its 13-billion-year journey to
Hubble's sensitive detectors.

"It's quite amazing that we are able to look back across 13 billion years
of time. We're looking at galaxies that have already evolved from smaller
precursors, but it's only a few hundred million years after the formation
of the first stars," Illingworth said.

If the Milky Way is a galactic senior citizen, then these galaxies are
toddlers or preschoolers. For now, researchers are unable to detect the
even smaller infant galaxies that must have merged to form these first
bright galaxies.

But the seeds of those first galaxies can be seen in the the cosmic
microwave background radiation, measured most recently and accurately by
the Wilkinson Microwave Anisotropy Probe (WMAP), which shows slight
fluctuations of density in a remarkably homogeneous universe about 400,000
years after the Big Bang.

"Very early in the evolution of the universe, everything was very smooth.
But over time the universe became more and more clumpy as gravity pulled
more matter into the denser areas," Bouwens said. "Our observations of
early galaxies allow us to measure how fast the universe was evolving from
smaller to larger clumps."

Detection of the very first galaxies to form will be possible with the
successor to Hubble, the James Webb Space Telescope, currently planned for
launch in 2013, Illingworth said.

Additional information about the search for the first galaxies is
available on the web at
http://firstgalaxies.ucolick.org/

Note to reporters: You may contact Illingworth at (831) 459-2843 and
Bouwens at (831) 459-5276.


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