  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.
