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This paper presents the Hubble Ultra Deep Field (HUDF), a one million second exposure of an 11 square minute-of-arc region in the southern sky with the Hubble Space Telescope. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB~29 for point sources. The image contains at least 10,000 objects presented here as a catalog. Few if any galaxies at redshifts greater than ~4 resemble present day spiral or elliptical galaxies. Using the Lyman break dropout method, we find 504 B-dropouts, 204 V-dropouts, and 54 i-dropouts. Using these samples that are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6. The highest redshift samples show that star formation was already vigorous at the earliest epochs that galaxies have been observed, less than one billion years after the Big Bang.Comment: 44 pages, 18 figures, to appear in the Astronomical Journal October 200

We combine HST imaging from the GEMS (Galaxy Evolution from Morphologies and SEDs) survey with photometric redshifts from COMBO-17 to explore the evolution of disk-dominated galaxies since z P1:1. The sample is composed of all GEMS galaxies with Sérsic indices n < 2:5, derived from fits to the galaxy images. We account fully for selection effects through careful analysis of image simulations; we are limited by the depth of the redshift and HST data to the study of galaxies with M V P À20, or equivalently, log M/M ð Þk 10. We find strong evolution in the magnitude-size scaling relation for galaxies with M V P À20, corresponding to a brightening of $1 mag arcsec À2 in rest-frame V band by z $ 1. Yet disks at a given absolute magnitude are bluer and have lower stellar mass-to-light ratios at z $ 1 than at the present day. As a result, our findings indicate weak or no evolution in the relation between stellar mass and effective disk size for galaxies with log M/M ð Þk 10 over the same time interval. This is strongly inconsistent with the most naive theoretical expectation, in which disk size scales in proportion to the halo virial radius, which would predict that disks are a factor of 2 denser at fixed mass at z $ 1. The lack of evolution in the stellar mass-size relation is consistent with an ''inside-out'' growth of galaxy disks on average (galaxies increasing in size as they grow more massive), although we cannot rule out more complex evolutionary scenarios.