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 $\mathrm{log}{M}_{* }\geqslant 10.3$ , 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 $\mathrm{log}{M}_{* }\lt 11$ , the relationship is consistent with ${R}_{e}\propto {\left(1+{z}_{\mathrm{form}}\right)}^{-1}$ , 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 $\mathrm{log}{M}_{* }\gt 11$ , 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 $\mathrm{log}{M}_{* }\gt 11$ , 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 (Σ_{1 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.