Chemical Abundances of Early Quiescent Galaxies: New Observations and Modeling Impacts

Abstract Recent stellar chemical abundance measurements of a handful of z ∼ 2 quiescent galaxies have suggested these galaxies exhibit a remarkably strong α -enhancement compared to their local and intermediate-redshift counterparts. This apparent chemical evolution following quenching suggests that even the innermost regions of massive early-type galaxies may have experienced substantial mixing of stars in mergers, challenging a purely inside-out growth model. However, larger samples are needed to determine whether a high α -enhancement ([Mg/Fe] ≈0.5) is common in z ∼ 2 quiescent galaxies, and a comparative analysis is needed to determine whether it is consistently inferred using different stellar population synthesis models. We report age and stellar chemical abundance measurements for a sample of four gravitationally lensed quiescent galaxies at z ∼ 2.1–2.65 based on Magellan/FIRE spectroscopy. For three of these galaxies we constrain the α -enhancement, and in two cases we measure high values comparable to earlier results when the spectra are analyzed consistently. We also find that the choice of modeling approach can exert a significant effect on the measured abundances. This model dependence can be partly, but not entirely, explained by the complex abundance patterns of α -elements in galaxies, which has been observed at lower redshifts and in one z ∼ 2 quiescent galaxy. Our investigation highlights the importance of independently varying abundance of α -elements when fitting the spectra of such galaxies. Observations with JWST will soon deliver precise and spatially resolved abundances of these and other quiescent galaxies at cosmic noon, opening a new window into their evolution.