Stellar kinematics of z ∼ 2 galaxies and the inside-out growth of quiescent galaxies

Abstract

Using stellar kinematics measurements, we investigate the growth of massive, quiescent galaxies from z ∼ 2 to today. We present X-Shooter spectra from the UV to NIR and dynamical mass measurements of five quiescent massive (>1011 M⊙) galaxies at z ∼ 2. This triples the sample of z > 1.5 galaxies with well-constrained (δσ < 100 km s-1) velocity dispersion measurements. From spectral population synthesis modeling we find that these galaxies have stellar ages that range from 0.5 to 2 Gyr, with no signs of ongoing star formation. We measure velocity dispersions (290-450 km s-1) from stellar absorption lines and find that they are 1.6-2.1 times higher than those of galaxies in the Sloan Digital Sky Survey at the same mass. Sizes are measured using GALFIT from Hubble Space Telescope Wide Field Camera 3 H160 and UDS K-band images. The dynamical masses correspond well to the spectral energy distribution based stellar masses, with dynamical masses that are ∼15% higher. We find that M*/Mdyn may decrease slightly with time, which could reflect the increase of the dark matter fraction within an increasing effective radius. We combine different stellar kinematic studies from the literature and examine the structural evolution from z ∼ 2 to z ∼ 0: we confirm that at fixed dynamical mass, the effective radius increases by a factor of ∼2.8, and the velocity dispersion decreases by a factor of ∼1.7. The mass density within one effective radius decreases by a factor of ∼20, while within a fixed physical radius (1 kpc) it decreases only mildly (factor of ∼2). When we allow for an evolving mass limit by selecting a population of galaxies at fixed number density, a stronger size growth with time is found (factor of ∼4), velocity dispersion decreases by a factor of ∼1.4, and interestingly, the mass density within 1 kpc is consistent with no evolution. This finding suggests that massive quiescent galaxies at z ∼ 2 grow inside out, consistent with the expectations from minor mergers. © 2013. The American Astronomical Society. All rights reserved.