Evidence for a supermassive black hole in NGC 3115

Stellar rotation velocities and velocity dispersions along the major and minor axes of the edge-on S0 galaxy NGC 3115 have been measured in effective seeing σ_*_ = 0. 42"-0. 69" with the Canada-France-Hawaii Telescope. Also, V-band surface photometry has been obtained in seeing σ_*_ = 0. 33"-0. 48". The kinematic data show an unresolved rise in rotation velocity to V = 165 +/- 4 km s^-1^ at 2" and a 62% increase in velocity dispersion at r 50 much larger than those of old stellar populations. This is a robust result independent of uncertainties in the unprojected kinematics or brightness distribution. Maximum entropy dynamical models are used to investigate the effects of velocity anisotropy. These also fail to fit the kinematics unless there is a rise in M/LV_ toward the center. The smallest central dark mass that gives a good fit to the observations is M ~ 1 x 10⁸^ Mₛun_. However, the tensor virial theorem suggests that the bulge is nearly isotropic. Then the observations imply that NGC 3115 contains a central dark object, possibly a black hole, of mass M ~1 to 2 x 10⁹^ Mₛun_.