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 . \textcolor blackWe find that early formation redshifts are correlated with high stellar-mass surface densities , \log \Sigma _ { 1 } / ( \hbox { $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 minimum formation redshift : all such objects in our sample have z _ { 50 } > 2.9 . \textcolor black 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 .