  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.
