Quiescent galaxies in the 3d-hst survey: Spectroscopic confirmation of a large number of galaxies with relatively old stellar populations at z ∼ 2

Quiescent galaxies at z ∼ 2 have been identified in large numbers based on rest-frame colors, but only a small number of these galaxies have been spectroscopically confirmed to show that their rest-frame optical spectra show either strong Balmer or metal absorption lines. Here, we median stack the rest-frame optical spectra for 171 photometrically quiescent galaxies at 1.4 < z < 2.2 from the 3D-HST grism survey. In addition to Hβ (λ4861 Å), we unambiguously identify metal absorption lines in the stacked spectrum, including the G band (λ4304 Å), Mg I (λ5175 Å), and Na I (λ5894 Å). This finding demonstrates that galaxies with relatively old stellar populations already existed when the universe was ∼3 Gyr old, and that rest-frame color selection techniques can efficiently select them. We find an average age of Gyr when fitting a simple stellar population to the entire stack. We confirm our previous result from medium-band photometry that the stellar age varies with the colors of quiescent galaxies: the reddest 80% of galaxies are dominated by metal lines and have a relatively old mean age of Gyr, whereas the bluest (and brightest) galaxies have strong Balmer lines and a spectroscopic age of Gyr. Although the spectrum is dominated by an evolved stellar population, we also find [O III] and Hβ emission. Interestingly, this emission is more centrally concentrated than the continuum with erg s-1, indicating residual central star formation or nuclear activity.