2003ApJ...585L..93Y

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 Hubble Space Telescope Advanced Camera for Surveys (ACS) coming on-line, we now have a 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. We find 30 plausible z≃6 candidates, all of which have signal-to-noise ratios greater than 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 four 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 α=-1.6. At the very least, our result suggests that galaxies with L<L^* do exist in significant numbers at z≃6 and that they could be the major sources that contributed the reionizing photons.