On central black holes in ultra-compact dwarf galaxies
Context. The dynamical mass-to-light (M/L) ratios of massive ultra-compact dwarf galaxies (UCDs) are about 50% higher than predicted by stellar population models. Aims. Here we investigate the possibility that these apparently elevated M/L ratios of UCDs are caused by a central black hole (BH) that heats up the internal motion of stars. We focus on a sample of ∼50 extragalactic UCDs from the literature for which velocity dispersions and structural parameters have been measured. Methods. To be self-consistent in our BH mass estimates, we first redetermine the dynamical masses and M/L ratios of our sample UCDs, using up-to-date distance moduli and a consistent treatment of aperture and seeing effects. On average, the homogeneously redetermined dynamical mass and M/L ratios agree to within 5% with previous literature results. We calculate the ratio Ψ= (M/L)dyn/(M/L)pop between the dynamical and the stellar population M/L for an assumed age of 13 Gyr. Ψ > 1 indicates an elevated dynamical M/L ratio, suggesting dark mass on top of a canonical stellar population of old age. For all UCDs with Ψ > 1w e estimate the mass of a hypothetical central black hole needed to reproduce the observed integrated velocity dispersion Results. Massive UCDs (M > 10 7 M� )h ave an average Ψ= 1.7 ± 0.2, implying notable amounts of dark mass in them. We find that, on average, central BH masses of 10–15% of the UCD mass can explain these elevated dynamical M/L ratios. The implied BH masses