We report results from our z \simeq 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 \leq z \leq 6.5 ( or z \simeq 6 for short ) candidates to m _ { AB } ( z _ { 850 } ) = 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 \simeq 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 \simeq 6 candidates to m _ { AB } ( z _ { 850 } ) = 29.2 mag suggests that the faint-end slope of the galaxy luminosity function ( LF ) at this redshift is likely between \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 \simeq 6 .
We also searched for z \simeq 6.5 –7 candidates using the UDF NICMOS data , and have found four candidates to J _ { 110 } = 27.2 mag .
However , the infrared colors of three candidates can not be easily explained by galaxies in this redshift range .
We tentatively derive an upper limit to the cumulative surface density of galaxies at z \simeq 7 of 0.36 per arcmin ^ { 2 } to J _ { 110 } = 26.6 mag , which suggest a noticeable drop in the LF amplitude from z \simeq 6 to z \simeq 7 .