2019ApJ...880L..31K

The chemical composition of galaxies has been measured out to z ∼ 4. However, nearly all studies beyond z ∼ 0.7 are based on strong-line emission from H II regions within star-forming galaxies. Measuring the chemical composition of distant quiescent galaxies is extremely challenging, as the required stellar absorption features are faint and shifted to near-infrared wavelengths. Here, we present ultradeep rest-frame optical spectra of five massive quiescent galaxies at z ∼ 1.4, all of which show numerous stellar absorption lines. We derive the abundance ratios [Mg/Fe] and [Fe/H] for three out of five galaxies; the remaining two galaxies have too young luminosity-weighted ages to yield robust measurements. Similar to lower-redshift findings, [Mg/Fe] appears positively correlated with stellar mass, while [Fe/H] is approximately constant with mass. These results may imply that the stellar mass-metallicity relation was already in place at z ∼ 1.4. While the [Mg/Fe]-mass relation at z ∼ 1.4 is consistent with the z < 0.7 relation, [Fe/H] at z ∼ 1.4 is ∼0.2 dex lower than at z < 0.7. With a [Mg/Fe] of 0.44_–0.07^+0.08 the most massive galaxy may be more α-enhanced than similar-mass galaxies at lower redshift, but the offset is less significant than the [Mg/Fe] of 0.6 previously found for a massive galaxy at z = 2.1. Nonetheless, these results combined may suggest that [Mg/Fe] in the most massive galaxies decreases over time, possibly by accreting low-mass, less α-enhanced galaxies. A larger galaxy sample is needed to confirm this scenario. Finally, the abundance ratios indicate short star formation timescales of 0.2-1.0 Gyr.