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

Abstract

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 $=7.6_{-1.6}^{+2.0}$ Gyr; $[Z/\text{H}]=-0.95_{-0.10}^{+0.12}$; $[\hat{I}\pm /\text{Fe}]=0.34_{-0.12}^{+0.10}$). Assuming a Kroupa stellar initial mass function, the stellar population parameters and luminosity ($M_{F814W}=-12.13\pm 0.06$ mag) yield a stellar mass of $M_{\ast }=3.9_{-0.6}^{+0.9}\times 10^6{M}_{\odot }$, which we also find to be consistent with near-infrared data. Via mass modelling, with our new measurements of velocity dispersion ($\ddot{I}{ƒ}_{ap}=27\pm 2$ km s${}^{-1}$) and size ($R_e=20.9\pm 1.0$ pc), we obtain an elevated dynamical-to-stellar mass ratio $M_{dyn}/M_{\ast }=8.2$ (with a range $5.6\le M_{dyn}/M_{\ast }\le 11.2$). 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.

16 pages, 5 figures, accepted for publication in MNRAS