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 similar to -21 to M-1500 similar to -18) from z similar to 7 to z similar to 4. The sample contains similar to 400 z similar to 4, similar to 120 z similar to 5, similar to 60 z similar to 6, and 36 z similar to 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 similar to -18 sources at z similar to 4). At z similar to 4, we study the stacked SEDs over a range of 5 mag reaching down to similar 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 greater than or similar to 5, we find blue [3.6]-[4.5] colors similar to-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 similar to 2x. 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 similar to 4 x 10(6) to similar to 2 x 10(7) M-circle dot Mpc(-3)).