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