Blue Rest-frame UV-optical Colors in z ∼ 8 Galaxies from GREATS: Very Young Stellar Populations at ∼650 Myr of Cosmic Time

Abstract

Deep rest-optical observations are required to accurately constrain the stellar populations of z ∼ 8 galaxies. Due to significant limitations in the availability of such data for statistically complete samples, observational results have been limited to modest numbers of bright or lensed sources. To revolutionize the present characterization of z ∼ 8 galaxies, we exploit the ultradeep (∼27 mag, 3 σ) Spitzer/Infrared Array Camera (IRAC) 3.6 and 4.5 μm data, probing the rest-frame optical at z ∼ 8, over ∼200 arcmin ² of the GOODS fields from the recently completed GOODS Re-ionization Era wide-Area Treasury from Spitzer program (GREATS), combined with observations in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS)/Ultra Deep Survey (UDS) and CANDELS/Cosmic Evolution Survey (COSMOS) fields. We stacked ≳100 z ∼ 8 Lyman-break galaxies in four bins of UV luminosity ( M _UV ∼ −20.7 to −18.4 mag) and study their H ₁₆₀ − [3.6] and [3.6]–[4.5] colors. We find young ages (≲100 Myr) for the three faintest stacks, inferred from their blue H ₁₆₀ − [3.6] ∼ 0 mag colors, consistent with a negative Balmer break. Meanwhile, the redder H ₁₆₀ − [3.6] color seen in the brightest stack is suggestive of slightly older ages. We explored the existence of a correlation between the UV luminosity and age, and find either no trend or fainter galaxies being younger. The stacked SEDs also exhibit very red [3.6]–[4.5] ∼ 0.5 mag colors, indicative of intense [O iii]+H β nebular emission and star formation rate (SFR). The correspondingly high specific SFRs, sSFR ≳10 Gyr ⁻¹, are consistent with recent determinations at similar redshifts and higher luminosities, and support the coevolution between the sSFR and the specific halo mass accretion rate.