Abstract

We present an analysis of an H-160-selected photometric catalog of galaxies in the Hubble Ultra-Deep Field, using imaging from the WFC3/IR camera on the Hubble Space Telescope in combination with archival ultraviolet, optical, and near-infrared imaging. Using these data, we measure the spectral energy distributions of similar to 1500 galaxies to a limiting H-160 magnitude of 27.8, from which we fit photometric redshifts and stellar population estimates for all galaxies with well-determined Spitzer IRAC fluxes, allowing for the determination of the cumulative mass function within the range 1 z 6. By selecting samples of galaxies at a constant cumulative number density, we are able to explore the coevolution of stellar masses and star formation rates (SFRs) for progenitor galaxies and their descendants from z similar to 6. We find a steady increase in the SFRs of galaxies at constant number density from z similar to 6 to z similar to 3, accompanied by gradually declining specific star formation rates (sSFRs) during this same period. The peak epoch of star formation is also found to shift to later times for galaxies with increasing number densities, in agreement with the expectations from cosmic downsizing. The observed SFRs can fully account for the mass growth to z similar to 2 among galaxies with cumulative number densities greater than 10(-3.5) Mpc(-3). For galaxies with a lower constant number density (higher mean mass), we find the observed stellar masses are similar to three times greater than that which may be accounted for by the observed star formation alone at late times, implying that growth from mergers plays an important role at z 2. We additionally observe a decreasing sSFR, equivalent to approximately one order of magnitude, from z similar to 6 to z similar to 2 among galaxies with number densities less than 10(-3.5) Mpc(-3), along with significant evidence that at any redshift the sSFR is higher for galaxies at higher number density. The combination of these findings can qualitatively explain the previous findings of a specific star formation rate plateau at high redshift. Tracing the evolution of the fraction of quiescent galaxies for samples matched in cumulative number density over this redshift range, we find no unambiguous examples of quiescent galaxies at z > 4.