First Results from the VIRIAL Survey: The Stellar Content of UVJ-selected Quiescent Galaxies at 1.5 lt z lt 2 from KMOS

We investigate the stellar populations of 25 massive galaxies (${\rm log} [{{M}_{*}}/{{M}_{\odot }}]\geqslant 10.9$) at $1.5\lt z\lt 2$ using data obtained with the K-band Multi-Object Spectrograph (KMOS) on the ESO VLT. Targets were selected to be quiescent based on their broadband colors and redshifts using data from the 3D-HST grism survey. The mean redshift of our sample is $\bar{z}=1.75$, where KMOS YJ-band data probe age- and metallicity-sensitive absorption features in the rest-frame optical, including the G-band, Fe i, and high-order Balmer lines. Fitting simple stellar population models to a stack of our KMOS spectra, we derive a mean age of $1.03_{-0.08}^{+0.13}$ Gyr. We confirm previous results suggesting a correlation between color and age for quiescent galaxies, finding mean ages of $1.22_{-0.19}^{+0.56}$ Gyr and $0.85_{-0.05}^{+0.08}$ Gyr for the reddest and bluest galaxies in our sample. Combining our KMOS measurements with those obtained from previous studies at $0.2\lt z\lt 2$ we find evidence for a 2–3 Gyr spread in the formation epoch of massive galaxies. At $z\lt 1$ the measured stellar ages are consistent with passive evolution, while at $1\lt z\lesssim 2$ they appear to saturate at ~1 Gyr, which likely reflects changing demographics of the (mean) progenitor population. By comparing to star formation histories inferred for "normal" star-forming galaxies, we show that the timescales required to form massive galaxies at $z\gtrsim 1.5$ are consistent with the enhanced α-element abundances found in massive local early-type galaxies.