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 restframe 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 (λ4861Å), we unambiguously identify metal absorption lines in the stacked spectrum, including the G-band (λ4304Å), Mg I (λ5175Å), and Na I (λ5894Å). 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 Hβ emission. Interestingly, this emission is more centrally concentrated than the continuum with L OIII = 1.7 ± 0.3 × 10 40 erg s −1 , indicating residual central star formation or nuclear activity.

SPECTROSCOPIC CONFIRMATION OF THE RICHz= 1.80 GALAXY CLUSTER JKCS 041 USING THE WFC3 GRISM: ENVIRONMENTAL TRENDS IN THE AGES AND STRUCTURE OF QUIESCENT GALAXIES

“…The grayscale shows the field distribution for galaxies drawn from the NMBS survey (see Section 5.1) that have z = 1.8 ± 0.2 and M * > 10 10.6 M . The solid line divides the quiescent and star-forming selection regions, while the dashed line shows the partition between bluer and redder quiescent galaxies used by Whitaker et al (2013). Median color uncertainties are illustrated by the error bar.…”

“…We construct composite spectra that reveal age-sensitive stellar absorption lines at high signal-to-noise for the first time in such a distant cluster. Using these, we investigate the mean stellar age both as a function of mass within the cluster, and relative to similar field galaxies whose composite spectrum was constructed by Whitaker et al (2013) using 3D-HST grism data. The 17 spectroscopically confirmed cluster members in the continuum-selected sample (H 160 < 23.3), which is approximately mass-limited (M * 10 10.6 M , Section 4.3) and confined to R < R 500 ≈ 500 kpc, form the basis for the following comparisons.…”

“…Second, we avoid the rest-frame near-infrared where model uncertainties related to the TP-AGB phase can influence the derived ages around 1 Gyr. Third, we are able to make a homogeneous comparison to coeval, quiescent galaxies in the field, whose composite continuum-normalized spectrum was measured by Whitaker et al (2013) using 3D-HST survey data.…”

“…For further details and a revised estimate of the age scatter based on our spectroscopic data, we refer to Andreon et al (2014). Whitaker et al (2013) recently constructed composite spectra of 171 quiescent field galaxy observed in the 3D-HST grism 10 Given the ratio of the median stellar masses entering our two bins, we estimate a velocity dispersion ratio of Δ log σ ≈ 0. survey. This presents an interesting opportunity to compare quenched field and clusters galaxies at the same early epoch.…”

“…For the first time in such a distant cluster, the quality of our spectra is sufficient to measure age-sensitive stellar absorption features and derive mean stellar ages as a function of galaxy mass. Additionally, through a comparison with composite spectra assembled by Whitaker et al (2013) based on 3D-HST data, we are able to compare the stellar ages of quenched galaxies in JKCS 041 and the field near the same epoch. We demonstrate that although the fraction of quiescent systems in the cluster is elevated, the mean ages of these galaxies do not differ appreciably from the field sample.…”