  Recent photometric studies have shown that early-type galaxies at fixed
stellar mass were smaller and denser at earlier times. In this paper we assess
that finding by deriving the dynamical mass of such a compact quiescent galaxy
at z=1.8. We have obtained a high-quality spectrum with full UV-NIR wavelength
coverage of galaxy NMBS-C7447 using X-Shooter on the VLT. We determined a
velocity dispersion of 294 +- 51 km/s. Given this velocity dispersion and the
effective radius of 1.64 +- 0.15 kpc (as determined from HST-WFC3 F160W
observations) we derive a dynamical mass of 1.7 +- 0.5 x 10^11 Msun. Comparison
of the full spectrum with stellar population synthesis models indicates that
NMBS-C774 has a relatively young stellar population (0.40 Gyr) with little or
no star formation and a stellar mass of ~1.5 x 10^11 Msun. The dynamical and
photometric stellar mass are in good agreement. Thus, our study supports the
conclusion that the mass densities of quiescent galaxies were indeed higher at
earlier times, and this earlier result is not caused by systematic measurement
errors. By combining available spectroscopic measurements at different
redshifts, we find that the velocity dispersion at fixed dynamical mass was a
factor of ~1.8 higher at z=1.8 compared to z=0. Finally, we show that the
apparent discrepancies between the few available velocity dispersion
measurements at z>1.5 are consistent with the intrinsic scatter of the
mass-size relation.
