MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. I -
  Evolution of Structural and Dynamical Properties
  We present deep near-infrared spectra for a sample of 24 quiescent galaxies
in the redshift range 1.5 < z < 2.5 obtained with the MOSFIRE spectrograph at
the W. M. Keck Observatory. In conjunction with a similar dataset we obtained
in the range 1 < z < 1.5 with the LRIS spectrograph, we analyze the kinematic
and structural properties for 80 quiescent galaxies, the largest
homogeneously-selected sample to date spanning 3 Gyr of early cosmic history.
Analysis of our Keck spectra together with measurements derived from associated
HST images reveals increasingly larger stellar velocity dispersions and smaller
sizes to redshifts beyond z~2. By classifying our sample according to Sersic
indices, we find that among disk-like systems the flatter ones show a higher
dynamical to stellar mass ratio compared to their rounder counterparts which we
interpret as evidence for a significant contribution of rotational motion. For
this subset of disk-like systems, we estimate that V/sigma, the ratio of the
circular velocity to the intrinsic velocity dispersion, is a factor of two
larger than for present-day disky quiescent galaxies. We use the velocity
dispersion measurements also to explore the redshift evolution of the dynamical
to stellar mass ratio, and to measure for the first time the physical size
growth rate of individual systems over two distinct redshift ranges, finding a
faster evolution at earlier times. We discuss the physical origin of this
time-dependent growth in size in the context of the associated reduction of the
systematic rotation.
