THE EVOLUTION OF THE GALAXY REST-FRAME ULTRAVIOLET LUMINOSITY FUNCTION OVER THE FIRST TWO BILLION YEARS

Abstract

We present a robust measurement and analysis of the rest-frame ultraviolet\n(UV) luminosity function at z=4-8. We use deep Hubble Space Telescope imaging\nover the CANDELS/GOODS fields, the Hubble Ultra Deep Field and the Year 1\nHubble Frontier Field deep parallel observations. These surveys provides an\neffective volume of 0.6-1.2 x 10^6 Mpc^3 over this epoch, allowing us to\nperform a robust search for faint (M_UV=-18) and bright (M_UV < -21) galaxies.\nWe select candidate galaxies using a well-tested photometric redshift technique\nwith careful screening of contaminants, finding a sample of 7446 galaxies at\n3.5<z<8.5, with >1000 galaxies at z~6-8. We measure the luminosity function\nusing a Markov Chain Monte Carlo analysis to measure robust uncertainties. At\nthe faint end our results agree with previous studies, yet we find a higher\nabundance of UV-bright galaxies at z>6, with M* ~ -21 at z>5, different than\nthat inferred based on previous trends at lower redshift. At z=8, a single\npower-law provides an equally good fit to the UV luminosity function, while at\nz=6 and 7, an exponential cutoff at the bright-end is moderately preferred. We\ncompare to semi-analytical models, and find that the lack of evolution in M* is\nconsistent with models where the impact of dust attenuation on the bright-end\nof the luminosity function decreases at higher redshift. We measure the\nevolution of the cosmic star-formation rate density, correcting for dust\nattenuation, and find that it declines as (1+z)^(-4.3 +/- 0.5) at z>4,\nconsistent with observations at z>9. Our observations are consistent with a\nreionization history that starts at z>10, completes at z>6, and reaches a\nmidpoint (x_HII = 0.5) at 6.7<z<9.4. Finally, our observations predict that the\nabundance of bright z=9 galaxies is likely higher than previous constraints,\nthough consistent with recent estimates of bright z~10 galaxies. [abridged]\n