Evidence for a central dark mass in NGC 4594 (the Sombrero galaxy)

Stellar rotation velocities and velocity dispersions along the major and minor axes of NGC 4594 have been measured in effective seeing σ_*_ = 0. 38"-0. 47" with the Canada-France-Hawaii Telescope. There is a clear kinematic signature of a nuclear disk of stars superposed on the bulge. Apart from its large size, this is similar to the nucleus of M31. It rotates rapidly: the apparent rotation curve reaches an inner maximum of V = 231+/-7 km s^-1^ at r= 5. 0". The apparent velocity dispersion falls from σ = 250+/-7 km s^-1^ at the center to 181+/-6 km s^-1^ at r = 3. 7". The latter is smaller than the local σ~ 240 km s^-1^ in the bulge. After subtraction of bulge light, the true outer dispersion of the nucleus is 50. This is much larger than normal for old stellar populations. Velocity anisotropies are not a major uncertainty because of the rapid rotation. Exploration of model parameter space shows that the central rise in M/LV_ is a robust result independent of the assumed forms of V (r) and σ (r). Therefore the observations imply that NGC 4594 contains a central dark object possibly a black hole of mass M ~10⁹^ Mₛun_.