First results from the virial survey: The stellar content of UVJ-selected quiescent galaxies at 1.5 < z < 2 from KMOS

We investigate the stellar populations of 25 massive galaxies (log[M_z.ast;/M_⊙] ≥ 10.9) at 1.5 < z < 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 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 (Formula presented)Gyr. We confirm previous results suggesting a correlation between color and age for quiescent galaxies, finding mean ages of (Formula presented) Gyr and (Formula presented)Gyr for the reddest and bluest galaxies in our sample. Combining our KMOS measurements with those obtained from previous studies at 0.2 < z < 2 we find evidence for a 2-3 Gyr spread in the formation epoch of massive galaxies. At z < 1 the measured stellar ages are consistent with passive evolution, while at 1 < z ≲ 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 ≳ 1.5 are consistent with the enhanced α-element abundances found in massive local early-type galaxies.