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-*/M-circle dot] >= 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) over bar = 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<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 less than or similar to 2 they appear to saturate at similar to 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 greater than or similar to 1.5 are consistent with the enhanced alpha-element abundances found in massive local early-type galaxies.