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 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 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.