1211.3424 (Jesse van de Sande et al.)

Stellar kinematics of z~2 galaxies and the inside-out growth of quiescent galaxies    [PDF]

Jesse van de Sande, Mariska Kriek, Marijn Franx, Pieter G. van Dokkum, Rachel Bezanson, Rychard J. Bouwens, Ryan F. Quadri, Hans-Walter Rix, Rosalind E. Skelton

Using stellar kinematics measurements, we investigate whether massive, quiescent galaxies were denser at z~2 than they are today. We present X-Shooter spectra from the UV to NIR and dynamical mass measurements of 5 quiescent massive (>10^11 Msun) galaxies at z~2. This triples the sample of z>1.5 galaxies with well constrained (delta sigma<100 km/s) velocity dispersion measurements. From spectral population synthesis modeling we find that these galaxies have stellar ages that range from 0.5-2 Gyr, with no sign of on-going star formation. We measure velocity dispersions (290-450 km/s) and find that they are 1.6-2.1 times higher than those of galaxies in the SDSS at fixed mass. Sizes are measured using GALFIT from HST-WFC3 H_160 and UDS K-band images. The dynamical masses correspond well to the SED-based stellar masses, with dynamical masses that are ~15% higher. We find that M_*/M_dyn may decrease slightly with time, which could reflect the increase of the dark matter fraction within an 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 with time. The mass density within one effective radius decreases by a factor of ~21, while within a fixed physical radius (1 kpc) it decreases only mildly (factor of ~2.3). 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 in an inside-out matter, consistent with the expectations from minor mergers.

View original: http://arxiv.org/abs/1211.3424