z ~ 7-10 Galaxies in the HUDF and GOODS Fields: UV Luminosity Functions

Abstract

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 zgtrsim 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 phiv* at z ~ 7 are -19.8 +/- 0.4 mag and 1.1^{+ 1.7}_-0.7 × 10^-3 Mpc^-3, respectively. Our search results for z ~ 9 J-dropouts set a 1 σ lower limit on M_UV* of -19.6 mag assuming that phiv* 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 zgtrsim 5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cutoff at the bright end.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 7235, 7817, 9425, 9575, 9797, 9803, 9978, 9979, 10189, 10339, 10340, 10403, 10530, 10632, 10872, and 11082. Observations have been carried out using the Very Large Telescope at the ESO Paranal Observatory under Program ID LP168.A-0485. Based in part on data collected at the Subaru Telescope and obtained from SMOKA, which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan.