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

Continued photometric monitoring of the gravitational lens system 0957+561A,B in the g and r bands with the Apache Point Observatory (APO) 3.5 m telescope during 1996 shows a sharp g band event in the trailing (B) image light curve at the precise time predicted in an earlier paper. The prediction was using gravitational lenses and some other possible implications and uses of the 0957+561A,B light curves.

We detect three (plus one less certain) z 850 -dropout sources in two separate fields (HUDF and NICP34) of our UDF05 HST NICMOS images. These z ∼ 7 Lyman-Break Galaxy (LBG) candidates allow us to constrain the Luminosity Function (LF) of the star forming galaxy population at those epochs. By assuming a change in only M * and adopting a linear evolution in redshift, anchored to the measured values at z ∼ 6, the best-fit evolution coefficient is found to be 0.43 ± 0.19 mag per unit redshift (0.36 ± 0.18, if including all four candidates), which provides a value of M * (z = 7.2) = −19.7 ± 0.3. This implies a drop of the luminosity density in LBGs by a factor of ∼ 2 − 2.5 over the ∼ 170 Myr that separate z ∼ 6 and z ∼ 7, and a steady evolution for the LBG LF out to z ∼ 7, at the same rate that is observed throughout the z ∼ 3 to 6 period. This puts a strong constraint on the star-formation histories of z ∼ 6 galaxies, whose ensemble star-formation rate density must be lower by a factor 2 at ∼170 Myr before the epoch at which they are observed. In particular, a large fraction of stars in the z ∼ 6 LBG population must form at redshifts well above z ∼ 7. The rate of ionizing photons produced by the LBG population decreases consistently with the decrease in the cosmic star formation rate density. Extrapolating this steady evolution of the LF out to higher redshifts, we estimate that galaxies would be able to reionize the universe by z ∼ 6, provided that the faint-end slope of the z > 7 LF steepens to α ∼ −1.9, and that faint galaxies, with luminosities below the current detection limits, contribute a substantial fraction of the required ionizing photons. This scenario gives however an integrated optical depth to electron scattering that is ∼ 2σ below the WMAP-5 measurement. Therefore, altogether, our results indicate that, should galaxies be the primary contributors to reionization, either the currently detected evolution of the galaxy population slows down at z 7, or the LF evolution must be compensated by a decrease in metallicity and a corresponding increase in ionization efficiency at these early epochs.

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We present grism spectra taken with the Advanced Camera for Surveys (ACS) to identify 29 red sources with (i 775 À z 850 ) ! 0:9 in the Hubble Ultra Deep Field (HUDF). Of these, 23 are found to be galaxies at redshifts between z ¼ 5:4 and 6.7, identified by the break at 1216 8 due to intergalactic medium (IGM) absorption; two are late-type dwarf stars with red colors; and four are galaxies with colors and spectral shapes similar to dust-reddened or old galaxies at redshifts z % 1 2. This constitutes the largest uniform, flux-limited sample of spectroscopically confirmed galaxies at such faint fluxes (z 850 27:5). Many are also among the most distant spectroscopically confirmed galaxies (at redshifts up to z ¼ 6:7). We find a significant overdensity of galaxies at redshifts z ¼ 5:9 AE 0:2. Nearly two-thirds of the galaxies in our sample (15/23) belong to this peak. Taking into account the selection function and the redshift sensitivity of the survey, we get a conservative overdensity of at least a factor of 2 along the line of sight. The galaxies found in this redshift peak are also localized in the plane of the sky in a nonrandom manner, occupying about half of the ACS chip. Thus the volume overdensity is a factor of 4. The star formation rate derived from detected sources in this overdense region is sufficient to reionize the local IGM.