Abstract
We report results from our z 5.5-7 galaxy search in the Hubble Space Telescope Ultra Deep Field (UDF). Using the 400 orbit of Advanced Camera for Surveys data, we found 108 plausible 5.5 ≤ z ≤ 6.5 (or z 6 for short) candidates to mAB(z850) = 30.0 mag. The contamination to the sample, due to either image artifacts or known types of astronomical objects, is likely negligible. The inferred surface densities of z 6 galaxies are consistent with our earlier predictions from mAB(z850) = 26.5 to 28.5 mag. After correcting for detection incompleteness, the counts of z 6 candidates to mAB(z850) = 29.2 mag suggest that the faint-end slope of the galaxy luminosity function (LF) at this redshift is likely between α = -1.8 and -1.9, which is sufficient to account for the entire Lyman photon budget necessary to complete the reionization of the universe at z 6. We also searched for z 6.5-7 candidates using the UDF Near-Infrared Camera and Multi-Object Spectrometer data and have found four candidates to a magnitude of J110 = 27.2. However, the infrared colors of three candidates cannot be easily explained by galaxies in this redshift range. We tentatively derive an upper limit to the cumulative surface density of galaxies at z 7 of 0.36 arcmin-2 to a magnitude of J110 = 26.6, which suggests a noticeable drop in the LF amplitude from z 6 to z 7.
Published Version
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