Reconstructing the Assembly of Massive Galaxies. I. The Importance of the Progenitor Effect in the Observed Properties of Quiescent Galaxies at z ≈ 2

Abstract

We study the relationship between the morphology and star formation history (SFH) of 361 quiescent galaxies (QGs) at redshift 〈z obs〉 ≈ 2, with stellar mass , selected with the UVJ technique. Taking advantage of panchromatic photometry covering the rest-frame UV-to-NIR spectral range ( ≈40 bands), we reconstruct the nonparametric SFH of the galaxies with the fully Bayesian SED fitting code Prospector. We find that the half-light radius R e , observed at z obs, depends on the formation redshift of the galaxies, z form, and that this relationship depends on M *. At , the relationship is consistent with , in line with the expectation that the galaxies’ central density depends on the cosmic density at the time of their formation, i.e., the “progenitor effect.” At , the relationship between R e and z form flattens, suggesting that mergers become increasingly important for the size growth of more massive galaxies after they quenched. We also find that the relationship between z form and galaxy compactness similarly depends on M *. While no clear trend is observed for QGs with , lower-mass QGs that formed earlier, i.e., with larger z form, have larger central stellar-mass surface densities, both within the R e (Σ e ) and central 1 kpc (Σ1 kpc), and also larger M 1 kpc/M *, the fractional mass within the central 1 kpc. These trends between z form and compactness, however, essentially disappear if the progenitor effect is removed by normalizing the stellar density with the cosmic density at z form. Our findings highlight the importance of reconstructing the SFH of galaxies before attempting to infer their intrinsic structural evolution.