How elevated is the dynamical-to-stellar mass ratio of the ultra-compact
  dwarf S999?
  Here we present new Keck ESI high-resolution spectroscopy and deep archival
HST/ACS imaging for S999, an ultra-compact dwarf in the vicinity of M87, which
was claimed to have an extremely high dynamical-to-stellar mass ratio. Our data
increase the total integration times by a factor of 5 and 60 for spectroscopy
and imaging, respectively. This allows us to constrain the stellar population
parameters for the first time (simple stellar population equivalent age
$=7.6^{+2.0}_{-1.6}$ Gyr; $[Z/\textrm{H}]=-0.95^{+0.12}_{-0.10}$;
$[\alpha/\textrm{Fe}]=0.34^{+0.10}_{-0.12}$). Assuming a Kroupa stellar initial
mass function, the stellar population parameters and luminosity
($M_{F814W}=-12.13\pm0.06$ mag) yield a stellar mass of
$M_*=3.9^{+0.9}_{-0.6}\times10^6 M_{\odot}$, which we also find to be
consistent with near-infrared data. Via mass modelling, with our new
measurements of velocity dispersion ($\sigma_{ap}=27\pm2$ km s$^{-1}$) and size
($R_e=20.9\pm1.0$ pc), we obtain an elevated dynamical-to-stellar mass ratio
$M_{dyn}/M_*=8.2$ (with a range $5.6\le M_{dyn}/M_* \le 11.2$). Furthermore, we
analyse the surface brightness profile of S999, finding only a small excess of
light in the outer parts with respect to the fitted S\'ersic profile, and a
positive colour gradient. Taken together these observations suggest that S999
is the remnant of a much larger galaxy that has been tidally stripped. If so,
the observed elevated mass ratio may be caused by mechanisms related to the
stripping process: the existence of an massive central black hole or internal
kinematics that are out of equilibrium due to the stripping event. Given the
observed dynamical-to-stellar mass ratio we suggest that S999 is an ideal
candidate to search for the presence of an overly massive central black hole.
