Spitzer UltRa Faint SUrvey Program (SURFS UP). II. IRAC-Detected
  Lyman-Break Galaxies at 6 < z < 10 Behind Strong-Lensing Clusters
  We study the stellar population properties of the IRAC-detected $6 \lesssim z
\lesssim 10$ galaxy candidates from the Spitzer UltRa Faint SUrvey Program
(SURFS UP). Using the Lyman Break selection technique, we find a total of 16
new galaxy candidates at $6 \lesssim z \lesssim 10$ with $S/N \geq 3$ in at
least one of the IRAC $3.6\mu$m and $4.5\mu$m bands. According to the best mass
models available for the surveyed galaxy clusters, these IRAC-detected galaxy
candidates are magnified by factors of $\sim 1.2$--$5.5$. We find that the
IRAC-detected $6 \lesssim z \lesssim 10$ sample is likely not a homogeneous
galaxy population: some are relatively massive (stellar mass as high as $4
\times 10^9\,M_{\odot}$) and evolved (age $\lesssim 500$ Myr) galaxies, while
others are less massive ($M_{\text{stellar}}\sim 10^8\,M_{\odot}$) and very
young ($\sim 10$ Myr) galaxies with strong nebular emission lines that boost
their rest-frame optical fluxes. We identify two Ly$\alpha$ emitters in our
sample from the Keck DEIMOS spectra, one at $z_{\text{Ly}\alpha}=6.76$ (in
RXJ1347) and one at $z_{\text{Ly}\alpha}=6.32$ (in MACS0454). We show that IRAC
$[3.6]-[4.5]$ color, when combined with photometric redshift, can be used to
identify galaxies likely with strong nebular emission lines within certain
redshift windows.
