We identify 73 z \sim 7 and 59 z \sim 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 density at z \sim 7 and z \sim 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 WFC3/IR and ACS data available over the HUDF09 ( \sim 29.4 AB mag , 5 \sigma ) , two nearby HUDF09 fields ( \sim 29 AB mag , 5 \sigma , 14 arcmin ^ { 2 } ) and the wider area ERS ( \sim 27.5 AB mag , 5 \sigma , \sim 40 arcmin ^ { 2 } ) .
The application of strict optical non-detection criteria ensures the contamination fraction is kept low ( just \sim 7 % in the HUDF ) .
This very low value includes a full assessment of the contamination from lower redshift sources , photometric scatter , AGN , spurious sources , low mass stars , and transients ( e.g. , SNe ) .
From careful modeling of the selection volumes for each of our search fields we derive luminosity functions for galaxies at z \sim 7 and z \sim 8 to < -18 AB mag .
The faint-end slopes \alpha at z \sim 7 and z \sim 8 are uncertain but very steep at \alpha = -2.01 \pm 0.21 and \alpha = -1.91 \pm 0.32 , respectively .
Such steep slopes contrast to the local \alpha \sim - 1.4 and may even be steeper than that at z \sim 4 where \alpha = -1.73 \pm 0.05 .
With such steep slopes ( \alpha \lesssim - 1.7 ) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization .
The star formation rate densities derived from these new z \sim 7 and z \sim 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 \sim 40 % higher at z < 7 .
This suggests that ( 1 ) the stellar mass densities inferred from the Spitzer IRAC photometry are reasonably accurate and ( 2 ) that the IMF at very high redshift may not be very different from that at later times .