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$(M_*/\rm{M_\odot})>10.8$. 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]=$-0.13\pm0.08$ with Bagpipes, and [Z/H]=$0.04\pm0.14$ with
Alf, a fall of $\sim0.2-0.3$ dex compared with the local Universe. We also
measure an iron abundance, [Fe/H] =$-0.18\pm0.08$, a fall of $\sim0.15$ dex
compared with the the local Universe. We measure the alpha enhancement via the
magnesium abundance, obtaining [Mg/Fe]=$0.23\pm$0.12, consistent with
similar-mass galaxies in the local Universe, indicating no evolution in the
average alpha enhancement of log$(M_*/\rm{M_\odot})=11$ quiescent galaxies over
the last $\sim8$ Gyr. This suggests the very high alpha enhancements recently
reported for several bright $z\sim1-2$ quiescent galaxies are due to their
extreme masses, log$(M_*/\rm{M_\odot})\gtrsim11.5$, rather than being typical
of the $z\gtrsim1$ 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, $z_\rm{form}=2.4^{+0.6}_{-0.3}$. Recent
studies have obtained varying average formation redshifts for $z\gtrsim1$
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
$z\gtrsim1$. Combining these with precise JWST $z>2$ quiescent-galaxy
stellar-mass functions will provide an independent test of formation redshifts
derived from spectral fitting.
