Searching for z ~ 6 Objects with the HST Advanced Camera for Surveys:   Preliminary Analysis of a Deep Parallel Field
  Recent results suggest that z ~ 6 marks the end of the reionization era. A large sample of objects at z ~ 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 ~ 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 ~ 6 candidates, all of which have S/N > 7 in the F850LP-band. The major source of contamination could be faint Galactic cool 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 <= z <= 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 ~ 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 ~ 6 and they could be the major sources that contributed to the reionizing photons. 
Haojing Yan, Rogier A. Windhorst and Seth H. Cohen