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 < z < 8. We use new wide-field near-infrared data in GOODS-S from the
CANDELS, HUDF09 and ERS programs to select galaxies via photometric redshift
measurements. Our sample consists of 2812 candidate galaxies at z > 3.5,
including 113 at z = 7 to 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. The median value of
beta evolves significantly from -1.82 (+0.00,-0.04) at z = 4, to -2.37
(+0.26,-0.06) at z = 7. Additionally, we find that faint galaxies at z = 7 have
beta = -2.68 (+0.39,-0.24) (~ -2.4 after correcting for observational bias);
this is redder than previous claims in the literature, and does not require
"exotic" stellar populations to explain their colors. This evolution can be
explained by an increase in dust extinction, with the timescale consistent with
low-mass AGB 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 A. 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 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 ~ 3, may extend out to z = 7 - 8.
