The Stellar Metallicities of Massive Quiescent Galaxies at 1.0 < z < 1.3 from KMOS plus VANDELS

We present a rest-frame UV-optical (lambda = 2500-6400 angstrom) stacked spectrum representative of massive quiescent galaxies at 1.0 < z < 1.3 with log(M-*/M-circle dot) > 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 similar to 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 similar to 0.15 dex compared with the local universe. We measure the alpha enhancement via the magnesium abundance, obtaining [Mg/Fe] = 0.23 +/- 0.12, consistent with galaxies of similar mass in the local universe, indicating no evolution in the average alpha enhancement of log(M-*/M-circle dot) similar to 11 quiescent galaxies over the last similar to 8 Gyr. This suggests the very high alpha enhancements recently reported for several bright z similar to 1-2 quiescent galaxies are due to their extreme masses, log(M-*/M-circle dot) greater than or similar to 11.5, in accordance with the well-known downsizing trend, rather than being typical of the z greater than or similar to 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(-0.4)(+0.6) Gyr, corresponding to a formation redshift z(form) =2.4(-0.3)(+0.6) z greater than or similar to 1 massive quiescent galaxies, and, as these studies report consistent metallicities, we identify models with different star formation histories as the most likely cause. Larger spectroscopic samples from upcoming ground-based instruments will provide precise constraints on ages and metallicities at z greater than or similar to 1. Combining these with precise stellar mass functions for z > 2 quiescent galaxies from the James Webb Space Telescope will provide an independent test of formation redshifts derived from spectral fitting.

Automated summary

We present the stacked spectrum of massive quiescent galaxies at 1.0 < z < 1.3, showing a decrease in metallicity and iron abundance compared to the local universe. The average alpha enhancement of galaxies with similar mass has not evolved over the past 8 billion years, suggesting that high alpha enhancements in recent bright quiescent galaxies are due to their extreme masses. Our findings are consistent with recent studies and provide independent verification of formation redshifts.