Candidates of z ~= 5.5-7 Galaxies in the Hubble Space Telescope Ultra Deep Field

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 m_AB(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 m_AB(z₈₅₀)=26.5 to 28.5 mag. After correcting for detection incompleteness, the counts of z~=6 candidates to m_AB(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.