UV-continuum Slopes of >4000 z ~ 4-8 Galaxies from the HUDF/XDF, HUDF09, ERS, CANDELS-South, and CANDELS-North Fields

Bouwens, R. J.; Illingworth, G. D.; Oesch, P. A.; Labbé, I.; van Dokkum, P. G.; Trenti, M.; Franx, M.; Smit, R.; Gonzalez, V.; Magee, D.


We measure the UV-continuum slope β for over 4000 high-redshift galaxies over a wide range of redshifts z ~ 4-8 and luminosities from the HST HUDF/XDF, HUDF09-1, HUDF09-2, ERS, CANDELS-N, and CANDELS-S data sets. Our new β results reach very faint levels at z ~ 4 (-15.5 mag: 0.006 Lz=3*), z ~ 5 (-16.5 mag: 0.014 Lz=3*), and z ~ 6 and z ~ 7 (-17 mag: 0.025 Lz=3*). Inconsistencies between previous studies led us to conduct a comprehensive review of systematic errors and develop a new technique for measuring β 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 β. We find that after correcting for the systematic errors (typically Δβ ~ 0.1-0.2) all β results at z ~ 7 from different groups are in excellent agreement. The mean β we measure for faint (-18 mag: 0.1 Lz=3*) z ~ 4, z ~ 5, z ~ 6, and z ~ 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 β values are redder than we have reported in the past, but bluer than other recent results. Our previously reported trend of bluer β's at lower luminosities is confirmed, as is the evolution to bluer β's at high redshifts. β appears to show only a mild luminosity dependence faintward of M UV, AB ~ -19 mag, suggesting that the mean β asymptotes to ~-2.2 to -2.4 for faint z >= 4 galaxies. At z ~ 7, the observed β's suggest non-zero, but low dust extinction, and they agree well with values predicted in cosmological hydrodynamical simulations.

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