From Carbon to Cobalt: Chemical Compositions and Ages of z ∼ 0.7 Quiescent Galaxies

We present elemental abundance patterns (C, N, Mg, Si, Ca, Ti, V, Cr, Fe, Co, and Ni) for a population of 135 massive quiescent galaxies at z similar to 0.7 with ultra-deep rest-frame optical spectroscopy drawn from the LEGA-C survey. We derive average ages and elemental abundances in four bins of stellar velocity dispersion (sigma ( v )) ranging from 150-250 km s(-1) using a full-spectrum hierarchical Bayesian model. The resulting elemental abundance measurements are precise to 0.05 dex. The majority of elements, as well as the total metallicity and stellar age, show a positive correlation with sigma ( v ). Thus, the highest dispersion galaxies formed the earliest and are the most metal-rich. We find only mild or nonsignificant trends between [X/Fe] and sigma ( v ), suggesting that the average star formation timescale does not strongly depend on velocity dispersion. To first order, the abundance patterns of the z similar to 0.7 quiescent galaxies are strikingly similar to those at z similar to 0. However, at the lowest-velocity dispersions, the z similar to 0.7 galaxies have slightly enhanced N, Mg, Ti, and Ni abundance ratios and earlier formation redshifts than their z similar to 0 counterparts. Thus, while the higher-mass quiescent galaxy population shows little evolution, the low-mass quiescent galaxies population has grown significantly over the past 6 Gyr. Finally, the abundance patterns of both z similar to 0 and z similar to 0.7 quiescent galaxies differ considerably from theoretical prediction based on a chemical evolution model, indicating that our understanding of the enrichment histories of these galaxies is still very limited.