Rapid Evolution in the Most Luminous Galaxies During the First 900 Million Years

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  • 10 July 2006
Abstract
We take advantage of all available deep HST NICMOS data (~700 orbits) over the Extended CDF-South and HDF-North GOODS regions with overlapping ACS data to conduct a search for starbursting galaxies at z~7-8 using a z-dropout technique. We only find 1 object in our most conservative selection and 4 in a less conservative selection, but expect 10 and 17, respectively, if we assume no-evolution relative to the large samples available at z~6. This is inconsistent with no-evolution at >=99.9% confidence assuming simple Poissonian statistics (and >=99.4% confidence if we include the effects of large-scale structure) and indicates that the volume density of bright (~0.6-2.5 L*) galaxies at z~7-8 was much lower than at z~6, just 200 million years later. The discovery shows that luminous, massive galaxies are quite deficient 700 million years after recombination, and provides key evidence for hierarchical galaxy buildup at early times. (abridged)

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