The Rest-frame UV-to-optical Colors and Spectral Energy Distributions of z ~ 4-7 Galaxies

Abstract

We use the ultra-deep HUDF09 and the deep Early Release Science data from the HST WFC3/IR camera, along with the wide-area Spitzer/IRAC data from GOODS-S to derive spectral energy distributions (SEDs) of star-forming galaxies from the rest-frame UV to the optical over a wide luminosity range (M ₁₅₀₀ ~ -21 to M ₁₅₀₀ ~ -18) from z ~ 7 to z ~ 4. The sample contains ~400 z ~ 4, ~120 z ~ 5, ~60 z ~ 6, and 36 z ~ 7 galaxies. Median stacking enables the first comprehensive SED study of very faint high-z galaxies at multiple redshifts (e.g., [3.6] = 27.4 ± 0.1 AB mag for the M ₁₅₀₀ ~ -18 sources at z ~ 4). At z ~ 4, we study the stacked SEDs over a range of 5 mag reaching down to ~0.06L* _{z = 4}. We use all available SEDs and template fits to derive rest-frame UV-to-optical colors (U - V) at all redshifts and luminosities. We find that this color does not vary significantly with redshift at a fixed luminosity. The UV-to-optical color does show a weak trend with luminosity, becoming redder at higher luminosities. This is most likely due to dust. At z >~ 5, we find blue [3.6]-[4.5] colors ~ -0.3 mag that are most likely due to rest-frame optical emission lines contributing to the flux in the IRAC filter bandpasses. Such contributions would lower both ages and masses by ~2 ×. The scatter in our derived SEDs remains large, but the results are most consistent with a lack of any evolution in the SEDs with redshift at a given luminosity. The uniformity of the SEDs suggests a self-similar mode of evolution over a timespan from 0.7 Gyr to 1.5 Gyr after the big bang that encompasses very substantial growth in the stellar mass density in the universe (from ~4 × 10⁶ to ~2 × 10⁷ M _⊙ Mpc^-3).