Recent results suggest that z \simeq 6 marks the end of the reionization era .
A large sample of objects at z \simeq 6 , therefore , will be of enormous importance , as it will enable us to observationally determine the exact epoch of the reionization and the sources that are responsible for it .
With the HST Advanced Camera for Surveys ( ACS ) coming on line , we now have an unique opportunity to discover a significant number of objects at z \simeq 6 .
The pure parallel mode implemented for the Wide Field Camera ( WFC ) has greatly enhanced this ability .
We present our preliminary analysis of a deep ACS/WFC parallel field at |b| = 74.4 ^ { o } .
We find 30 plausible z \simeq 6 candidates , all of which have S / N > 7 in the F850LP-band .
The major source of contamination could be faint cool Galactic dwarfs , and we estimated that they would contribute at most 4 objects to our candidate list .
We derived the cumulative number density of galaxies at 6.0 \leq z \leq 6.5 as 2.3 arcmin ^ { -2 } to a limit of 28.0 mag in the F850LP-band , which is slightly higher than our prediction .
If this is not due to an underestimated contamination rate , it could possibly imply that the faint-end slope of the z \simeq 6 luminosity function is steeper than \alpha = -1.6 .
At the very least , our result suggests that galaxies with L < L ^ { * } do exist in significant number at z \simeq 6 , and that they could be the major sources that contributed the reionizing photons .