Elemental Abundances and Ages of z ∼ 0.7 Quiescent Galaxies on the Mass–Size Plane: Implication for Chemical Enrichment and Star Formation Quenching

We present elemental abundances and stellar population ages for 65 massive quiescent galaxies at 0.59 ≤ z ≤ 0.75 from the LEGA-C survey. The abundance patterns and ages, derived from full-spectrum modeling, are examined as a function of stellar mass (M*) and size (i.e., half-light radius, Re). We find that both [Mg/H] and [Fe/H] do not vary with stellar mass but are correlated with M*/Re for quiescent galaxies with M* > 1010.5 M⊙. Thus, at fixed mass, compact quiescent galaxies are on average more metal-rich. This result reinforces the picture that supernova feedback and gravitational potential regulate chemical enrichment. [Mg/Fe] does not vary with M* or M*/Re, but there is a marginal positive relation between age and mass. Our results support low-redshift findings that more massive galaxies form their stars at earlier times. However, in contrast to low-redshift studies, star formation timescale does not appear to depend on mass or size. We also compare the mass–[Fe/H] and mass–[Mg/H] relations to stacks of quiescent galaxies at z ∼ 0 and find that both relations increase by ∼0.2 dex over the past 7 Gyr. Furthermore, at z ∼ 0.7 we find a clear trend with age, such that older quiescent galaxies have lower metallicities. Both results can be explained by a chemical evolution model in which galaxies quench via gas removal. Future work, in particular with James Webb Space Telescope/NIRSpec, will extend this analysis to higher redshifts, allowing us to fully exploit abundance patterns to study the formation histories of quiescent galaxies.