Blue Rest-Frame UV-Optical Colors in z~8 Galaxies from GREATS: Very
  Young Stellar Populations at ~650 Myr of Cosmic Time
  Deep rest-optical observations are required to accurately constrain the
stellar populations of $z\sim8$ 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 $z\sim8$ galaxies, we exploit the
ultradeep ($\sim27$ mag, $3\sigma$) Spitzer/IRAC $3.6\mu$m and $4.5\mu$m data,
probing the rest-frame optical at $z\sim8$, over $\sim200$ 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 $\gtrsim100$ $z\sim8$ Lyman-Break
galaxies in four bins of UV luminosity ($M_\mathrm{UV}\sim -20.7$ to $-18.4$)
and study their $H_\mathrm{160}-[3.6]$ and $[3.6]-[4.5]$ colors. We find young
ages ($\lesssim100$ Myr) for the three faintest stacks, inferred from their
blue $H_\mathrm{160}-[3.6]\sim 0$ mag colors, consistent with a negative Balmer
break. Meanwhile, the redder $H_\mathrm{160}-[3.6]$ 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
$[3.6]-[4.5]\sim0.5$ mag colors, indicative of intense [OIII]+H$\beta$ nebular
emission and SFR. The correspondingly high specific star-formation rates,
sSFR$\gtrsim10$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.
