REQUIEM-2D: Spatially Resolved Stellar Populations from HST 2D Grism Spectroscopy

We present a novel Bayesian methodology to jointly model photometry and deep Hubble Space Telescope (HST) 2d grism spectroscopy of high-redshift galaxies. Our requiem2d code measures both unresolved and resolved stellar populations, ages, and star-formation histories (SFHs) for the ongoing REQIUEM (REsolving QUIEscent Magnified) Galaxies Survey, which targets strong gravitationally lensed quiescent galaxies at z~2. We test the accuracy of \texttt{requiem2d} using a simulated sample of massive galaxies at z~2 from the Illustris cosmological simulation and find we recover the general trends in SFH and median stellar ages. We further present a pilot study for the REQUIEM Galaxies Survey: MRG-S0851, a quintuply-imaged, massive ($\log M_* / M_\odot = 11.02 \pm 0.04$) red galaxy at $z=1.883\pm 0.001$. With an estimated gravitational magnification of $\mu = 5.7^{+0.4}_{-0.2}$, we sample the stellar populations on 0.6 kpc physical size bins. The global mass-weighted median age is constrained to be $1.8_{-0.2}^{+0.3}$ Gyr, and our spatially resolved analysis reveals that MRG-S0851 has a flat age gradient in the inner 3 kpc core after taking into account the subtle effects of dust and metallicity on age measurements, favoring an early formation scenario. The analysis for the full REQUIEM-2D sample will be presented in a forthcoming paper with a beta-release of the requiem2d code.