From Carbon to Cobalt: Chemical compositions and ages of $z\sim 0.7$ quiescent galaxies

Abstract

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 $z\sim 0.7$ 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 kms${}^{-1}$ to 250 kms${}^{-1}$ using a full-spectrum hierarchical Bayesian model. The resulting elemental abundance measurements are precise to 0.05dex. 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 non-significant 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 $z\sim 0.7$ quiescent galaxies are strikingly similar to those at $z\sim 0$. However, at the lowest velocity dispersions the $z\sim 0.7$ galaxies have slightly enhanced N, Mg, Ti, and Ni abundance ratios and earlier formation redshifts than their $z\sim 0$ counterparts. Thus, while the higher-mass quiescent galaxy population shows little evolution, the low-mass quiescent galaxies population has grown significantly over the past six billion years. Finally, the abundance patterns of both $z\sim 0$ and $z\sim 0.7$ 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.

24 pages, 12 figures, accepted for publication in ApJ

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