X-shooter Spectroscopy and HST Imaging of 15 Ultra Massive Quiescent
  Galaxies at $z\gtrsim2$
  We present a detailed analysis of a large sample of spectroscopically
confirmed ultra-massive quiescent galaxies
(${\rm{log}}(M_{\ast}/M_{\odot})\sim11.5$) at $z\gtrsim2$. This sample
comprises 15 galaxies selected in the COSMOS and UDS fields by their bright
K-band magnitudes and followed up with VLT/X-shooter spectroscopy and HST/WFC3
$H_{F160W}$ imaging. These observations allow us to unambiguously confirm their
redshifts ascertain their quiescent nature and stellar ages, and to reliably
assess their internal kinematics and effective radii. We find that these
galaxies are compact, consistent with the high mass end of the mass-size
relation for quiescent galaxies at $z=2$. Moreover, the distribution of the
measured stellar velocity dispersions of the sample is consistent with the most
massive local early-type galaxies from the MASSIVE Survey showing that
evolution in these galaxies, is dominated by changes in size. The HST images
reveal, as surprisingly high, that $40\ \%$ of the sample have tidal features
suggestive of mergers and companions in close proximity, including three
galaxies experiencing ongoing major mergers. The absence of velocity dispersion
evolution from $z=2$ to $0$, coupled with a doubling of the stellar mass, with
a factor of four size increase and the observed disturbed stellar morphologies
support dry minor mergers as the primary drivers of the evolution of the
massive quiescent galaxies over the last 10 billion years.
