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

Abstract

We investigate the stellar populations of 25 massive, galaxies ($\log [M_{\ast }/M_{\odot }]\ge 10.9$) 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 $\bar{z}=1.75$, 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 $0.2<z<2$ 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 $1<z\lesssim 2$ they appear to saturate at $\sim$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 $\alpha$-element abundances found in massive local early-type galaxies.

6 pages, 5 figures, accepted for publication in ApJL

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