The stellar metallicities of massive quiescent galaxies at 1.0 < z < 1.3 from KMOS+VANDELS

We present a rest-frame UV-optical stacked spectrum representative of massive quiescent galaxies at 1.0<z<1.3 with log(M∗/M⊙)>10.8. The stack is constructed using VANDELS survey data, combined with new KMOS observations. We apply two independent full-spectral-fitting approaches, measuring a total metallicity, [Z/H]=−0.13±0.08 with Bagpipes, and [Z/H]=0.04±0.14 with Alf, a fall of ∼0.2−0.3 dex compared with the local Universe. We also measure an iron abundance, [Fe/H] =−0.18±0.08, a fall of ∼0.15 dex compared with the the local Universe. We measure the alpha enhancement via the magnesium abundance, obtaining [Mg/Fe]=0.23±0.12, consistent with similar-mass galaxies in the local Universe, indicating no evolution in the average alpha enhancement of log(M∗/M⊙)=11 quiescent galaxies over the last ∼8 Gyr. This suggests the very high alpha enhancements recently reported for several bright z∼1−2 quiescent galaxies are due to their extreme masses, log(M∗/M⊙)≳11.5, rather than being typical of the z≳1 population. The metallicity evolution we observe with redshift (falling [Z/H], [Fe/H], constant [Mg/Fe]) is consistent with recent studies. We recover a mean stellar age of 2.5+0.6−0.4 Gyr, corresponding to a formation redshift, zform=2.4+0.6−0.3. Recent studies have obtained varying average formation redshifts for z≳1 massive quiescent galaxies, and, as these studies report consistent metallicities, we identify different star-formation-history models as the most likely cause. Larger spectroscopic samples from upcoming ground-based instruments will provide precise constraints on ages and metallicities at z≳1. Combining these with precise JWST z>2 quiescent-galaxy stellar-mass functions will provide an independent test of formation redshifts derived from spectral fitting.