Galaxies in the Hudf and Goods Fields, and Their 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 & 7 and constrain the UV LF within the first 700 Myrs. Our data set includes ∼ 23 arcmin of deep NICMOS J +H data and ∼ 248 arcmin of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find 8 ∼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 −0.7 × 10 −3 Mpc−3, respectively. Our search results for z ∼ 9 J-dropouts set a 1σ 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. The inferred brightening in M∗ UV with redshift (i.e., M∗ UV = (−21.02±0.09)+(0.36±0.08)(z−3.8)) matches the evolution expected in the halo mass function, if the mass-to-light ratio of halos evolves as ∼ (1 + z)−1. 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 cut-off at the bright end. Subject headings: galaxies: evolution — galaxies: high-redshift