HST Grism Observations of a Gravitationally Lensed Redshift 10 Galaxy
  We present deep spectroscopic observations of a Lyman-break galaxy candidate
(hereafter MACS1149-JD) at $z\sim9.5$ with the $\textit{Hubble}$ Space
Telescope ($\textit{HST}$) WFC3/IR grisms. The grism observations were taken at
4 distinct position angles, totaling 34 orbits with the G141 grism, although
only 19 of the orbits are relatively uncontaminated along the trace of
MACS1149-JD. We fit a 3-parameter ($z$, F160W mag, and Ly$\alpha$ equivalent
width) Lyman-break galaxy template to the three least contaminated grism
position angles using an MCMC approach. The grism data alone are best fit with
a redshift of $z_{\mathrm{grism}}=9.53^{+0.39}_{-0.60}$ ($68\%$ confidence), in
good agreement with our photometric estimate of
$z_{\mathrm{phot}}=9.51^{+0.06}_{-0.12}$ ($68\%$ confidence). Our analysis
rules out Lyman-alpha emission from MACS1149-JD above a $3\sigma$ equivalent
width of 21 \AA{}, consistent with a highly neutral IGM. We explore a scenario
where the red $\textit{Spitzer}$/IRAC $[3.6] - [4.5]$ color of the galaxy
previously pointed out in the literature is due to strong rest-frame optical
emission lines from a very young stellar population rather than a 4000 \AA{}
break. We find that while this can provide an explanation for the observed IRAC
color, it requires a lower redshift ($z\lesssim9.1$), which is less preferred
by the $\textit{HST}$ imaging data. The grism data are consistent with both
scenarios, indicating that the red IRAC color can still be explained by a 4000
\AA{} break, characteristic of a relatively evolved stellar population. In this
interpretation, the photometry indicate that a $340^{+29}_{-35}$ Myr stellar
population is already present in this galaxy only $\sim500~\mathrm{Myr}$ after
the Big Bang.
