CANDELS: THE EVOLUTION OF GALAXY REST-FRAME ULTRAVIOLET COLORS FROM z=8 TO 4

We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 less than or similar to z less than or similar to 8. We use new wide-field near-infrared data in the Great Observatories Origins Deep Survey-South field from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble Ultra Deep Field (HUDF) 2009, and Early Release Science programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z greater than or similar to 3.5, including 113 at z similar or equal to 7-8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models and measure the value of the UV spectral slope (beta) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on beta than other methods, as well as a reduced number of galaxies with catastrophically incorrect beta measurements (i.e.,. Delta beta > 1). We find that the median value of beta evolves significantly from -1.82(-0.04)(+0.00) at z = 4 to -2.37(-0.06)(+0.26) at z = 7. Additionally, we find that faint galaxies at z = 7 have beta = -2.68(-0.24)(+0.39) (similar to-2.4 after correcting for observational bias); this is redder than previous claims in the literature and does not require exotic stellar populations (e. g., very low metallicities or top-heavy initial mass functions) to explain their colors. This evolution can be explained by an increase in dust extinction, from low amounts at z = 7 to A(V) similar to 0.5 mag at z = 4. The timescale for this increase is consistent with low-mass asymptotic giant branch stars forming the bulk of the dust. We find no significant (<2 sigma) correlation between beta and M-UV when measuring M-UV at a consistent rest-frame wavelength of 1500 angstrom. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between beta and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have similarly red colors at each redshift, implying that dust can build up quickly in massive galaxies and that feedback is likely removing dust from low-mass galaxies at z >= 7. Thus, the stellar-mass-metallicity relation, previously observed up to z similar to 3, may extend out to z = 7-8.