z ∼ 7-10 galaxies in the HUDF and GOODS fields:: UV luminosity functions

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 greater than or similar to 7 and constrain the UV LF within the first 700 Myr. Our data set includes similar to 23 arcmin(2) of deep NICMOS J + H data and similar to 248 arcmin(2) of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find eighth < z > similar to 7.3 z-dropouts in our search fields, but no z similar to 9 J-dropout candidates. A careful consideration of a wide variety of different contaminants suggest an overall contamination level of just similar to 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 similar to 7 and z similar to 9. For a faint-end slope alpha - -1.74, our most likely values for M-UV* and phi* at z similar to 7 are -19.8 +/- 0.4 mag and 1.1(-0.7)(+1.7) x 10(-3) Mpc(-3), respectively. Our search results for z similar to 9 J-dropouts set a 1 sigma lower limit on M-UV* of -19.6 mag assuming that phi* and alpha 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 similar to 4, suggesting that the UV LF has undergone substantial evolution over this time period. No evolution is ruled out at 99% confidence from z similar to 7 to z similar to 6 and at 80% confidence from z similar to 9 to z similar to 7. We find that the mass-to-light ratio of halos evolves as similar to(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 greater than or similar to 5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cutoff at the bright end.