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 $z\sim 8$ 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 $z\sim 8$ galaxies, we exploit the ultradeep ($\sim 27$ mag, $3\sigma$) Spitzer/IRAC $3.6\mu$m and $4.5\mu$m data, probing the rest-frame optical at $z\sim 8$, over $\sim 200$ arcmin${}^2$ of the GOODS fields from the recently completed GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) program, combined with observations in the CANDELS UDS and COSMOS fields. We stacked $\gtrsim 100$ $z\sim 8$ Lyman-Break galaxies in four bins of UV luminosity ($M_{\mathrm{UV}}\sim -20.7$ to $-18.4$) and study their $H_{160}-[3.6]$ and $[3.6]-[4.5]$ colors. We find young ages ($\lesssim 100$ Myr) for the three faintest stacks, inferred from their blue $H_{160}-[3.6]\sim 0$ 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 $[3.6]-[4.5]\sim 0.5$ mag colors, indicative of intense [OIII]+H$\beta$ nebular emission and SFR. The correspondingly high specific star-formation rates, sSFR$\gtrsim 10$Gyr${}^{-1}$, are consistent with recent determinations at similar redshifts and higher luminosities, and support the co-evolution between the sSFR and the specific halo mass accretion rate.

Submitted to ApJ. Comments welcome. See Fig. 2 & 3 for the stacks, Fig. 4 & 6 for the colors/age and Fig. 8 for the sSFR evolution

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