ar X iv : a st ro - p h / 04 07 49 3 v 1 2 2 Ju l 2 00 4 Candidates of z ≃ 5 . 5 – 7 Galaxies in the HST Ultra Deep Field

We report results from our z ≃ 5.5–7 galaxy search in the HST Ultra Deep Field (UDF). Using the 400-orbit of ACS 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, either due to 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 suggests 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 NICMOS data, and have found four candidates to J110 = 27.2 mag. 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 per arcmin to J110 = 26.6 mag, which suggest a noticeable drop in the LF amplitude from z ≃ 6 to z ≃ 7. Subject headings: cosmology: observations — early universe — galaxies: highredshift — galaxies: luminosity function, mass function — galaxies: evolution