How elevated is the dynamical-to-stellar mass ratio of the ultracompact dwarf S999?

Here we present new Keck Echelle Spectrograph and Imager high-resolution spectroscopy and deep archival Hubble Space Telescope/Advanced Camera for Surveys imaging for S999, an ultracompact 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/H]=-0.95^{+0.12}_{-0.10}; [α /Fe]=0.34^{+0.10}_{-0.12}). Assuming a Kroupa stellar initial mass function, the stellar population parameters and luminosity (MF814W = -12.13 ± 0.06 mag) yield a stellar mass of M_*=3.9^{+0.9}_{-0.6}× 10^6 M_{⊙}, which we also find to be consistent with near-infrared data. Via mass modelling, with our new measurements of velocity dispersion (σap = 27 ± 2 km s-1) and size (Re = 20.9 ± 1.0 pc), we obtain an elevated dynamical-to-stellar mass ratio Mdyn/M* = 8.2 (with a range 5.6 ≤ Mdyn/M* ≤ 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 Sérsic 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 a 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.