Quiescent Galaxies in the 3D-HST Survey: Spectroscopic Confirmation of a Large Number of Galaxies with Relatively Old Stellar Populations at z ~ 2
Abstract
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 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 [O III] and Hβ emission. Interestingly, this emission is more centrally concentrated than the continuum with {L_{{O}\,\scriptsize{III}}}=1.7+/- 0.3\times 10^{40} erg s-1, indicating residual central star formation or nuclear activity.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2013
- DOI:
- 10.1088/2041-8205/770/2/L39
- arXiv:
- arXiv:1305.1943
- Bibcode:
- 2013ApJ...770L..39W
- Keywords:
-
- galaxies: evolution;
- galaxies: formation;
- galaxies: high-redshift;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 6 pages, 4 figures, accepted for publication in the Astrophysical Journal Letters