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}_⊙ }]≥slant 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≲ 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.

Based on observations obtained at the Very Large Telescope (VLT) of the European Southern Observatory (ESO), Paranal, Chile (ESO program IDs 092.A-0091, 093.A-0079, 093.A-0187, and 094.A-0287). This work is further based on observations taken by the 3D-HST Treasury Program (GO 12177 and 12328) with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.