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 (SFHs). SFHs are better constrained for higher redshift galaxies, observed closer to their formation and quenching epochs. Here we use "nonparametric" SFHs 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 Lyα 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 $z_{50}\simeq 5-8$. We find that early formation redshifts are correlated with high stellar-mass surface densities, $log{\Sigma }_1/\left(M_{\odot }{kpc}^{-2}\right)>10.25$, where Σ₁ is the stellar mass within 1 pkpc (proper kpc). Quiescent galaxies with the highest stellar-mass surface density, $log{\Sigma }_1/\left(M_{\odot }{kpc}^{-2}\right)>10.25$, show a minimum formation redshift: all such objects in our sample have $z_{50}>2.9$. Quiescent galaxies with lower surface density, $log{\Sigma }_1/\left(M_{\odot }{kpc}^{-2}\right)=9.5-10.25$, show a range of formation epochs ($z_{50}\simeq 1.5\text{--}8$), implying these galaxies experienced a range of formation and assembly histories. We argue that the surface density threshold $log{\Sigma }_1/\left(M_{\odot }{kpc}^{-2}\right)>10.25$ 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.