Evidence for Early Formation of the Most Compact Quiescent Galaxies at High Redshift Using Deep Hubble Grism Data Vince Estrada-Carpenter1 NASA FINESST Future Investigator Casey Papovich1, Iva Momcheva2, Gabe Brammer3, Raymond Simons2 and the CLEAR Collaboration The origin of the correlations between mass, morphology, and formation history in galaxies is diﬃcult to deﬁne, primarily due to uncertainties in galaxy star-formation histories, which are better constrained for higher redshift galaxies. Here we use a forward modeling technique on deep HST grism data (a method which can be applied to grism data from the Nancy Grace Roman Space Telescope) from the CLEAR (CANDELS Lyα Emission at Reionization) survey. We derive constraints on the formation and quenching timescales of quiescent galaxies at 0.7 < z < 2.5 using “non-parametric” star-formation histories. The galaxy formation redshifts, z 50 (deﬁned as the point where they formed 50% of their stellar mass) range from z50 ~ 2 (shortly prior to the observed epoch) up to z50 ~ 5-8. We ﬁnd that early formation redshifts are correlated with high stellar-mass surface densities, log(Σ1) > 10.25, where Σ1 is the stellar mass within 1 proper kpc. Quiescent galaxies with the highest stellar-mass surface density, log(Σ1) > 10.25, show a minimum formation redshift: all such objects in our sample have z50 > 2.9. Quiescent galaxies with lower surface density show a range of formation epochs (z50 ~ 1.5 - 8), implying these galaxies experienced a range of formation and assembly histories. We argue that the surface density threshold log(Σ1) > 10.25 uniquely identiﬁes galaxies that formed in the ﬁrst few Gyr after the big bang, and discuss the implications this has for galaxy formation models. Abstract Acknowledgment References VEC acknowledges support from the NASA Headquarters under the Future Investigators in NASA Earth and Space Science and Technology (FINESST) award 19-ASTRO19-0122, as well as support from the Hagler Institute for Advanced Study at Texas A&M University. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #14227. Support for program #14227 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The Most Compact Galaxies Formed at High Redshift Our SFHs Span Many Shapes Extended Galaxies Have Extended Star-Formation ●UVJ selected quiescent galaxies at 0.6 < z < 3.5 ●Selection criteria from Whitaker et al. 2011, prevents selection of dusty star-forming galaxies ●Selected ~ 100 massive quiescent galaxies We Select Quiescent Galaxies From CLEAR ●Main result of this work shows that galaxies with higher ∑1 measurements have higher formation redshifts ●The most compact galaxies all have z50 > 2.9 creating an envelope at the highest ∑1 , suggesting that the most compact galaxies can not form at lower redshifts ●The ﬁgures on the right show that this relationship monotonically rises and the standard deviation drops ●These results suggest that the most compact galaxies get their compact morphology from having formed in the early universe, when the universe was more dense ●Data: WFC3 G102 (CLEAR), WFC3 G141 (3D-HST, Momcheva et al 2016), photometry (3D-HST catalog, Skelton 2014) ●Fit all data simultaneously using forward modeling technique outlined in Estrada-Carpenter et al. 2019 and non-parametric star-formation histories (SFHs) from Leja et al. 19 ●SFHs (bottom panel in each subplot) span many shapes ○Use SFHs to measure z50 (formation redshift - when galaxy formed half its mass) We Quantify “Compactness” with ∑ ●Use ∑1 (stellar-mass surface density within 1 pkpc) to quantify how compact a galaxy is ●Being a in