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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 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 ≳ 3.5, including 113 at z ≃ 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 (β) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on β than other methods, as well as a reduced number of galaxies with catastrophically incorrect β measurements (i.e., Δβ > 1). We find that the median value of β 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 β = –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 (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 AV ∼ 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 (<2σ) correlation between β and MUV when measuring MUV at a consistent rest-frame wavelength of 1500 Å. 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 β 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 ∼ 3, may extend out to z = 7-8. [ABSTRACT FROM AUTHOR]

We present a rest-frame ultraviolet morphological analysis of 108 z ≃ 2.1 Lyα emitters (LAEs) in the Extended Chandra Deep Field South and compare it to a similar sample of 171 LAEs at z ≃ 3.1. Using Hubble Space Telescope images from the Galaxy Evolution from Morphology and SEDs survey, Great Observatories Origins Deep Survey, and Hubble Ultradeep Field, we measure size and photometric component distributions, where photometric components are defined as distinct clumps of UV-continuum emission. At both redshifts, >80% of LAEs have observed half-light radii <2 kpc, but the median half-light radius rises from 0.95 ± 0.04 kpc at z = 3.1 to 1.41 ± 0.14 kpc at z = 2.1. A similar evolution is seen in the sizes of individual rest-UV components, but there is no evidence for evolution in the number of multi-component systems. In the z = 2.1 sample, we see clear correlations between the size of an LAE and other physical properties derived from its spectral energy distribution (SED). LAEs are found to be larger for galaxies with higher stellar mass, star formation rate, and dust obscuration, but there is no evidence for a trend between equivalent width and half-light radius at either redshift. The presence of these correlations suggests that a wide range of objects are being selected by LAE surveys at z ∼ 2, including a significant fraction of objects for which a massive and moderately extended population of old stars underlies the young starburst giving rise to the Lyα emission. [ABSTRACT FROM AUTHOR]