Stellar Properties of z ~ 8 Galaxies in the Reionization Lensing Cluster
  Survey
  Measurements of stellar properties of galaxies when the universe was less
than one billion years old yield some of the only observational constraints of
the onset of star formation. We present here the inclusion of
\textit{Spitzer}/IRAC imaging in the spectral energy distribution fitting of
the seven highest-redshift galaxy candidates selected from the \emph{Hubble
Space Telescope} imaging of the Reionization Lensing Cluster Survey (RELICS).
We find that for 6/8 \textit{HST}-selected $z\gtrsim8$ sources, the $z\gtrsim8$
solutions are still strongly preferred over $z\sim$1-2 solutions after the
inclusion of \textit{Spitzer} fluxes, and two prefer a $z\sim 7$ solution,
which we defer to a later analysis. We find a wide range of intrinsic stellar
masses ($5\times10^6 M_{\odot}$ -- $4\times10^9$ $M_{\odot}$), star formation
rates (0.2-14 $M_{\odot}\rm yr^{-1}$), and ages (30-600 Myr) among our sample.
Of particular interest is Abell1763-1434, which shows evidence of an evolved
stellar population at $z\sim8$, implying its first generation of star formation
occurred just $< 100$ Myr after the Big Bang. SPT0615-JD, a spatially resolved
$z\sim10$ candidate, remains at its high redshift, supported by deep
\textit{Spitzer}/IRAC data, and also shows some evidence for an evolved stellar
population. Even with the lensed, bright apparent magnitudes of these $z
\gtrsim 8$ candidates (H = 26.1-27.8 AB mag), only the \textit{James Webb Space
Telescope} will be able further confirm the presence of evolved stellar
populations early in the universe.
