Candidates of z≃5.5-7 galaxies in the Hubble Space Telescope Ultra Deep Field

We report results from our z similar or equal to 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 less than or equal to z less than or equal to 6.5 (or z similar or equal to 6 for short) candidates to m(AB)(z(850)) = 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 similar or equal to 6 galaxies are consistent with our earlier predictions from m(AB)(z(850)) = 26.5 to 28.5 mag. After correcting for detection incompleteness, the counts of z similar or equal to 6 candidates to m(AB)(z(850)) = 29.2 mag suggest that the faint-end slope of the galaxy luminosity function (LF) at this redshift is likely between and alpha = -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 similar or equal to 6. We also searched for z similar or equal to 6.5-7 candidates using the UDF Near-Infrared Camera and Multi-Object Spectrometer data and have found four candidates to a magnitude of J(110) = 27.2. However, the infrared colors of three candidates cannot 110 be easily explained by galaxies in this redshift range. We tentatively derive an upper limit to the cumulative surface density of galaxies at z similar or equal to 7 of 0.36 arcmin(-2) to a magnitude of J(110) = 26.6, which suggests a noticeable drop in the LF amplitude from z similar or equal to 6 to z similar or equal to 7.