http://arxiv.org/abs/1810.00008
We investigate the stellar populations for a sample of 24 quiescent galaxies at 1.5 < z < 2.5 using deep rest-frame optical spectra obtained with Keck MOSFIRE. By fitting templates simultaneously to the spectroscopic and photometric data, and exploring a variety of star formation histories, we obtain robust measurements of median stellar ages and residual levels of star formation. After subtracting the stellar templates, the stacked spectrum reveals the H-alpha and [NII] emission lines, providing an upper limit on the ongoing star formation rate of 0.9 +/- 0.1 Msun/yr, thus confirming the quiescence of this population. By combining the MOSFIRE data to our sample of Keck LRIS spectra at lower redshift, we analyze in a consistent manner the quiescent population at 1 < z < 2.5, finding a tight relation between the stellar age and the rest-frame U-V and V-J colors, with a scatter of only 0.13 dex. Using this combined dataset of 79 spectroscopically studied galaxies, we determine an empirical calibration that can be used to estimate the age of quiescent galaxies given their UVJ colors. Applying this age – color relation to large, photometric samples, the number density of quiescent galaxies of various ages can be determined. We model the number density evolution and find evidence for two distinct quenching paths: a fast quenching that produces compact, spheroidal post-starburst systems residing in overdense regions; and a slow quenching of larger galaxies. Fast quenching accounts for about a fifth of the growth of the red sequence at z~1.4, and half at z~2.2. We conclude that fast quenching is triggered by dramatic events such as gas-rich mergers, while slow quenching is likely caused by a different physical mechanism.
S. Belli, A. Newman and R. Ellis
Tue, 2 Oct 18
34/84
Comments: 24 pages, 15 figures, submitted
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