REQUIEM-2D: A diversity of formation pathways in a sample of
  spatially-resolved massive quiescent galaxies at z~2
  REQUIEM-2D (REsolving QUIEscent Magnified galaxies with 2D grism
spectroscopy) is comprised of a sample of 8 massive ($\log M_*/M_\odot > 10.6$)
strongly lensed quiescent galaxies at $z\sim2$. REQUIEM-2D combines the natural
magnification from strong gravitational lensing with the high
spatial-resolution grism spectroscopy of \emph{Hubble Space Telescope} through
a spectrophotometric fit to study spatially resolved stellar populations. We
show that quiescent galaxies in the REQUIEM-2D survey have diverse formation
histories manifesting as a gradient in stellar ages, including examples of (1)
a younger central region supporting outside-in formation, (2) flat age
gradients that show evidence for both spatially-uniform early formation or
inside-out quenching, and (3) regions at a fixed radial distance having
different ages (such asymmetries cannot be recovered when averaging stellar
population measurements azimuthally). The typical dust attenuation curve for
the REQUIEM-2D galaxies is constrained to be steeper than Calzetti's law in the
UV and generally consistent with $A_V<1$. Combined together and accounting for
the different physical radial distances and formation time-scales, we find that
the REQUIEM-2D galaxies that formed earlier in the universe exhibit slow and
uniform growth in their inner core, whereas the galaxies that formed later have
rapid inner growth in their inner core with younger ages relative to the
outskirts. These results challenge the currently accepted paradigm of how
massive quiescent galaxies form, where the earliest galaxies are thought to
form most rapidly. Significantly larger samples close to the epoch of formation
with similar data quality and higher spectral resolution are required to
validate this finding.
