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 similar to 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 H beta (lambda 4861 angstrom), we unambiguously identify metal absorption lines in the stacked spectrum, including the G band (lambda 4304 angstrom), Mg I (lambda 5175 angstrom), and Na I (lambda 5894 angstrom). This finding demonstrates that galaxies with relatively old stellar populations already existed when the universe was similar to 3 Gyr old, and that rest-frame color selection techniques can efficiently select them. We find an average age of 1.3(-0.3)(+0.1) 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 1.6(-0.4)(+0.5) Gyr, whereas the bluest (and brightest) galaxies have strong Balmer lines and a spectroscopic age of 0.9(-0.1)(+0.2) Gyr. Although the spectrum is dominated by an evolved stellar population, we also find [O III] and H beta emission. Interestingly, this emission is more centrally concentrated than the continuum with L-OIII = 1.7 +/- 0.3 x 10(40) erg s(-1), indicating residual central star formation or nuclear activity.