CLEAR II: Evidence for Early Formation of the Most Compact Quiescent
  Galaxies at High Redshift
  The origin of the correlations between mass, morphology, quenched fraction,
and formation history in galaxies is difficult to define, primarily due to the
uncertainties in galaxy star-formation histories. Star-formation histories are
better constrained for higher redshift galaxies, observed closer to their
formation and quenching epochs. Here we use "non-parametric" star-formation
histories and a nested sampling method to derive constraints on the formation
and quenching timescales of quiescent galaxies at $0.7<z<2.5$. We model deep
HST grism spectroscopy and photometry from the CLEAR (CANDELS Lyman$-\alpha$
Emission at Reionization) survey. The galaxy formation redshifts, $z_{50}$
(defined as the point where they had formed 50\% of their stellar mass) range
from $z_{50}\sim 2$ (shortly prior to the observed epoch) up to $z_{50} \simeq
5-8$. \editone{We find that early formation redshifts are correlated with high
stellar-mass surface densities, $\log \Sigma_1 / (M_\odot\ \mathrm{kpc}^{-2})
>$10.25, where $\Sigma_1$ is the stellar mass within 1~pkpc (proper kpc).
Quiescent galaxies with the highest stellar-mass surface density, $\log\Sigma_1
/ (M_\odot\ \mathrm{kpc}^{-2}) > 10.25$, } show a \textit{minimum} formation
redshift: all such objects in our sample have $z_{50} > 2.9$. Quiescent
galaxies with lower surface density, $\log \Sigma_1 / (M_\odot\
\mathrm{kpc}^{-2}) = 9.5 - 10.25$, show a range of formation epochs ($z_{50}
\simeq 1.5 - 8$), implying these galaxies experienced a range of formation and
assembly histories. We argue that the surface density threshold
$\log\Sigma_1/(M_\odot\ \mathrm{kpc}^{-2})>10.25$ uniquely identifies galaxies
that formed in the first few Gyr after the Big Bang, and we discuss the
implications this has for galaxy formation models.
