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

This is the authors' abstract. We don't add key points for this paper.

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 <~100 km s^-1^. This and the shape of the isophotes show that the nucleus is a disk. To search for a central black hole, three-dimensional velocity and velocity dispersion fields are found that fit the observations after projection and seeing convolution. The simplest good model has V = 335 km s^-1^ and σ = 265r^-0.5^ km s^-1^ (100 km s^-1^ <= σ <= 400 km s^-1^) at r <= 8". In all acceptable models, large velocities and velocity gradients imply that the total mass-to-light ratio rises by more than a factor of 10 at r < 1" to values M/L_V_ > 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/L_V_ 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 ~10^9^ M_sun_.