Abstract

We identify 73 z similar to 7 and 59 z similar to 8 candidate galaxies in the reionization epoch, and use this large 26-29.4 AB mag sample of galaxies to derive very deep luminosity functions to - 18 AB mag and the star formation rate (SFR) density at z similar to 7 and z similar to 8 (just 800 Myr and 650 Myr after recombination, respectively). The galaxy sample is derived using a sophisticated Lyman-break technique on the full two-year Wide Field Camera 3/infrared (WFC3/IR) and Advanced Camera for Surveys (ACS) data available over the HUDF09 (similar to 29.4 AB mag, 5 sigma), two nearby HUDF09 fields (similar to 29 AB mag, 5 sigma, 14 arcmin(2)), and the wider area Early Release Science (similar to 27.5 AB mag, 5 sigma, similar to 40 arcmin(2)). The application of strict optical non-detection criteria ensures the contamination fraction is kept low (just similar to 7% in the HUDF). This very low value includes a full assessment of the contamination from lower redshift sources, photometric scatter, active galactic nuclei, spurious sources, low-mass stars, and transients (e.g., supernovae). From careful modeling of the selection volumes for each of our search fields, we derive luminosity functions for galaxies at z similar to 7 and z similar to 8 to - 18 AB mag. The faint-end slopes alpha at z similar to 7 and z similar to 8 are uncertain but very steep at alpha = -2.01 +/- 0.21 and alpha = -1.91 +/- 0.32, respectively. Such steep slopes contrast to the local alpha greater than or similar to -1.4 and may even be steeper than that at z similar to 4 where alpha = -1.73 +/- 0.05. With such steep slopes (alpha less than or similar to -1.7) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. The SFR densities derived from these new z similar to 7 and z similar to 8 luminosity functions are consistent with the trends found at later times (lower redshifts). We find reasonable consistency with the SFR densities implied from reported stellar mass densities being only similar to 40% higher at z 7. This suggests that (1) the stellar mass densities inferred from the Spitzer Infrared Array Camera (IRAC) photometry are reasonably accurate and (2) that the initial mass function at very high redshift may not be very different from that at later times.