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We measure the evolution of the specific star formation rate (sSFR = SFR/Mstellar) between redshift 4 and 6 to assess the reported “constant” sSFR at z > 2. We derive stellar masses and star formation rates (SFRs) for a large sample of 750 z ∼ 4-6 galaxies in the GOODS-S field by fitting stellar population models to their spectral energy distributions. Dust extinction is derived from the observed UV colors. We evaluate different star formation histories (SFHs, constant and rising with time) and the impact of optical emission lines. The SFR and Mstellar values are insensitive to whether the SFH is constant or rising. The derived sSFR is very similar (within 0.1 dex) in two Mstellar bins centered at 1 and 5 × 109M☼. The effect of emission lines was, however, quite pronounced. Assuming no contribution from emission lines, the sSFR for galaxies at 5 × 109M☼ evolves weakly at z > 2 (sSFR(z)∝(1 + z)0.6 ± 0.1), consistent with previous results. When emission lines are included in the rest-frame optical bands, consistent with the observed Infrared Array Camera [3.6] and [4.5] fluxes, the sSFR shows higher values at high redshift following sSFR(z)∝(1 + z)1.0 ± 0.1, i.e., the best-fit evolution shows a sSFR ∼2.3 × higher at z ∼ 6 than at z ∼ 2. This is, however, a substantially weaker trend than that found at z < 2 and even than that expected from current models for z > 2 (sSFR(z)∝(1 + z)2.5). Even accounting for emission lines, the observed sSFR(z) trends at z > 2 are still in tension with theoretical expectations. [ABSTRACT FROM AUTHOR]

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 (M1500 ∼ –21 to M1500 ∼ –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 M1500 ∼ –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 × 107M☼ Mpc–3). [ABSTRACT FROM AUTHOR]