September 13, 2006
Tracing 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 at the University of California, Santa Cruz, used NASA's 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.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, so there must have been a lot of merging of smaller galaxies during those 200 million years.
This panel shows four candidate galaxies that are likely to have redshifts of 7 and thus have emitted their light when the universe was just 750 million years old. 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.