Abstract

We measure the UV-continuum slope beta for over 4000 high-redshift galaxies over a wide range of redshifts z similar to 4-8 and luminosities from the HST HUDF/XDF, HUDF09-1, HUDF09-2, ERS, CANDELS-N, and CANDELS-S data sets. Our new beta results reach very faint levels at z similar to 4 (-15.5 mag: 0.006 L-z-3(*)), z similar to 5 (-16.5 mag: 0.014L(z-3)(*)), and z similar to 6 and z similar to 7 (-17 mag: 0.025L(z=3)(*)). Inconsistencies between previous studies led us to conduct a comprehensive review of systematic errors and develop a new technique for measuring beta that is robust against biases that arise from the impact of noise. We demonstrate, by object-by-object comparisons, that all previous studies, including our own and those done on the latest HUDF12 data set, suffered from small systematic errors in beta. We find that after correcting for the systematic errors (typically Delta beta similar to 0.1-0.2) all beta results at z similar to 7 from different groups are in excellent agreement. The mean beta we measure for faint (-18 mag: 0.1L(z=3)(*)) z similar to 4, z similar to 5, z similar to 6, and z similar to 7 galaxies is -2.03 +/- 0.03 +/- 0.06 (random and systematic errors), -2.14 +/- 0.06 +/- 0.06, -2.24 +/- 0.11 +/- 0.08, and -2.30 +/- 0.18 +/- 0.13, respectively. Our new beta values are redder than we have reported in the past, but bluer than other recent results. Our previously reported trend of bluer beta's at lower luminosities is confirmed, as is the evolution to bluer beta' s at high redshifts. beta appears to show only a mild luminosity dependence faintward of M-UV,M-AB similar to -19 mag, suggesting that the mean beta asymptotes to similar to -2.2 to -2.4 for faint z >= 4 galaxies. At z similar to 7, the observed beta's suggest non-zero, but low dust extinction, and they agree well with values predicted in cosmological hydrodynamical simulations.