paper

HST Grism Observations of a Gravitationally Lensed Redshift 10 Galaxy

arXiv:1709.03992 · doi:10.3847/1538-4357/aaa9c2

Abstract

We present deep spectroscopic observations of a Lyman-break galaxy candidate (hereafter MACS1149-JD) at with the Space Telescope () 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 (, F160W mag, and Ly 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 ( confidence), in good agreement with our photometric estimate of ( confidence). Our analysis rules out Lyman-alpha emission from MACS1149-JD above a equivalent width of 21 Å, consistent with a highly neutral IGM. We explore a scenario where the red /IRAC 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 Å break. We find that while this can provide an explanation for the observed IRAC color, it requires a lower redshift (), which is less preferred by the imaging data. The grism data are consistent with both scenarios, indicating that the red IRAC color can still be explained by a 4000 Å break, characteristic of a relatively evolved stellar population. In this interpretation, the photometry indicate that a Myr stellar population is already present in this galaxy only after the Big Bang.

arXiv commentAccepted to ApJ. This is the accepted version

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