2008ApJ...686..230B

We use all available deep optical ACS and near-IR data over both the HUDF and the two GOODS fields to search for star-forming galaxies at z≳7 and constrain the UV LF within the first 700 Myr. Our data set includes ∼23 arcmin² of deep NICMOS J+H data and ∼248 arcmin² of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find eight z∼7.3 z-dropouts in our search fields, but no z∼9 J-dropout candidates. A careful consideration of a wide variety of different contaminants suggest an overall contamination level of just ∼12% for our z-dropout selection. After performing detailed simulations to accurately estimate the selection volumes, we derive constraints on the UV LFs at z∼7 and z∼9. For a faint-end slope α=-1.74, our most likely values for M^*_UV and φ* at z∼7 are -19.8±0.4 mag and 1.1^+1.7_–0.7x10^–3/Mpc3, respectively. Our search results for z∼9 J-dropouts set a 1 σ lower limit on M^*_UV of -19.6 mag assuming that φ* and α are the same as their values at slightly later times. This lower limit on M^*_UV is 1.4 mag fainter than our best-fit value at z∼4, suggesting that the UV LF has undergone substantial evolution over this time period. No evolution is ruled out at 99% confidence from z∼7 to z∼6 and at 80% confidence from z∼9 to z∼7. We find that the mass-to-light ratio of halos evolves as ~(1+z)^–1 if we require that the observed brightening in M^*_UV with redshift [i.e., M^*_UV=(-21.02±0.09)+(0.36±0.08)(z-3.8)] be consistent with the expected evolution in the halo mass function. Finally, we consider the shape of the UV LF at z≳5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cutoff at the bright end.