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 ± 0.1ABmag for the M 1500 -18sources at z ∼ 4). At z ∼ 4, we study the stacked SEDs over a range of 5mag 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 ∼-0.3mag 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 0.7Gyr to 1.5Gyr 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). © 2012. The American Astronomical Society. All rights reserved.