CANDELS: The Evolution of Galaxy Rest-Frame Ultraviolet Colors from z = 8 to 4
(34 votes over all institutions)
We present an analysis of the rest-frame UV colors of galaxies at 4<z<8. We use new wide-field NIR data in GOODS-S from CANDELS, as well as existing data in the HUDF and ERS regions to select galaxies via their photometric redshifts. Our sample consists of 2812 candidate galaxies, with 113 candidate galaxies at z=7-8. We fit the observed SEDs to synthetic stellar population models and measure the value of the rest-frame UV spectral slope beta from the best-fit model spectrum. This results in a smaller average scatter on beta than other methods. We compute a median value of beta and its associated uncertainty in redshift bins from z=4-8, and we find that beta evolves significantly from -1.83 +/- 0.02 at z=4, to -2.37 +0.28/-0.06 at z=7. Additionally, we find that faint galaxies at z=7 have beta = -2.68 +0.44/-0.23 (~ -2.5 after correcting for observational bias); this value does not require very-low metallicities in these galaxies. This evolution can be explained by increased dust extinction, from low amounts at z=7 to A_v~0.5 mag at z=4. This may be due to the increased presence of low-mass AGB stars forming the bulk of the dust, as there is insufficient time for the expected progenitors of M 7. Finally, we examine whether there is any correlation between beta and the UV luminosity or stellar mass. We find that the existence of a correlation is highly dependent upon the definition of M_UV, and we find no evidence for a significant correlation when measuring M_UV at rest-frame 1500 A. We do find a strong correlation between beta and the stellar mass, in that more massive galaxies exhibit redder colors. The most massive galaxies have similarly red colors at all redshifts, implying that dust builds up quickly in massive galaxies, and that feedback is likely removing SNe-created dust from low-mass galaxies at z>7 [abridged].
