Abstract
We present deep spectroscopic observations of a Lyman-break galaxy candidate
(hereafter MACS1149-JD) at $z\sim10$ with the Hubble Space Telescope (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 rest-frame Lyman-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 $z_grism=9.53^+0.39_-0.60$ ($68\%$ confidence), in
good agreement with our photometric estimate of
$z_phot=9.51^+0.06_-0.12$ ($68\%$ confidence). Our analysis
rules out Lyman-alpha emission from MACS1149-JD above a $3\sigma$ rest-frame
equivalent width of 21 \AA, consistent with a highly neutral IGM. We explore
a scenario where the red 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 rather than a 4000 \AA break. We find that this can provide an
explanation for the observed IRAC photometry, but only with a probability of
0.05. Instead, the grism data add credence to the scenario that the red IRAC
color is best explained by a 4000 \AA break, characteristic of a relatively
evolved stellar population. In this interpretation, the photometry indicates
that a $340^+29_-35\, Myr$ stellar population is already present
in this galaxy only $\sim500\,Myr$ after the Big Bang.
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