The elemental abundances of quiescent galaxies in the LEGA-C survey: the (non-)evolution of [α/Fe] from z = 0.75 to z = 0

Abstract

We measure the [$\alpha$/Fe] abundances for 183 quiescent galaxies at z = 0.60 - 0.75 with stellar masses ranging 10.4 \leq log10 10.4 $\le$ log10 (M${}_{\ast }$ /M${}_{\odot }$) $\le$ 11.6 selected from the LEGA-C survey. We estimate [$\alpha$/Fe] from the ratio of the spectral indices Mgb ($\lambda \sim 5177$ Å) and Fe4383, compared to predictions of simple stellar population models. We find that 91% of quiescents in our sample have supersolar [$\alpha$/Fe], with an average value of [$\alpha$/Fe] = +0.24 $\pm$ 0.01. We find no significant correlation between [$\alpha$/Fe] and stellar metallicity, mass, velocity dispersion, and average formation time. Galaxies that formed the bulk of their stellar mass on time scales shorter than 1 Gyr follow the same [$\alpha$/Fe] distribution as those which formed on longer time scales. In comparison to local early-type galaxies and to stacked spectra of quiescent galaxies at z = 0.38 and z = 0.07, we find that the average [$\alpha$/Fe] has not changed between z = 0.75 and the present time. Our work shows that the vast majority of massive quiescent galaxies at z $\sim$ 0.7 are $\alpha$-enhanced, and that no detectable evolution of the average [$\alpha$/Fe] has taken place over the last $\sim$ 6.5 Gyr.

12 pages, 10 figures, 2 tables, 1 appendix