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arXiv · 1305.1943

Quiescent Galaxies in the 3D-HST Survey: Spectroscopic Confirmation of a Large Number of Galaxies with Relatively Old Stellar Populations at z~2

Katherine E. Whitaker·Pieter G. van Dokkum·Gabriel Brammer·Ivelina G. Momcheva·Rosalind Skelton·Marijn Franx·Mariska Kriek·Ivo Labbe·Mattia Fumagalli·Britt F. Lundgren·Erica J. Nelson·Shannon G. Patel·Hans-Walter Rix
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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 Hbeta (4861A), we unambiguously identify metal absorption lines in the stacked spectrum, including the G-band (4304A), Mg I (5175A), and Na I (5894A). 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 [OIII] and Hbeta 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.

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Katherine E. Whitaker, Pieter G. van Dokkum, Gabriel Brammer, Ivelina G. Momcheva, Rosalind Skelton, Marijn Franx, Mariska Kriek, Ivo Labbe, Mattia Fumagalli, Britt F. Lundgren, Erica J. Nelson, Shannon G. Patel, Hans-Walter Rix. 2013-05-10. Quiescent Galaxies in the 3D-HST Survey: Spectroscopic Confirmation of a Large Number of Galaxies with Relatively Old Stellar Populations at z~2. https://doi.org/10.1088/2041-8205%2F770%2F2%2Fl39

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