Using the MOSFIRE near-infrared multi-slit spectrograph on the Keck 1 Telescope , we have secured high signal-to-noise ratio absorption line spectra for six massive galaxies with redshift 2 < z < 2.5 .
Five of these galaxies lie on the red sequence and show signatures of passive stellar populations in their rest-frame optical spectra .
By fitting broadened spectral templates we have determined stellar velocity dispersions and , with broad-band HST and Spitzer photometry and imaging , stellar masses and effective radii .
Using this enlarged sample of galaxies we confirm earlier suggestions that quiescent galaxies at z > 2 have small sizes and large velocity dispersions compared to local galaxies of similar stellar mass .
The dynamical masses are in very good agreement with stellar masses ( \log M _ { \ast } / M _ { \mathrm { dyn } } = -0.02 \pm 0.03 ) , although the average stellar-to-dynamical mass ratio is larger than that found at lower redshift ( -0.23 \pm 0.05 ) .
By assuming evolution at fixed velocity dispersion , not only do we confirm a surprisingly rapid rate of size growth but we also consider the necessary evolutionary track on the mass-size plane and find a slope \alpha = \mathrm { d } \log \R / \mathrm { d } \log M _ { \ast } \gtrsim 2 inconsistent with most numerical simulations of minor mergers .
Both results suggest an additional mechanism may be required to explain the size growth of early galaxies .