Spectroscopic Evidence for a Supermassive Black Hole in Ngc 4486b

The stellar kinematics of the low-luminosity elliptical galaxy NGC 4486B have been measured in seeing σ∗ = 0. 22 with the Canada-France-Hawaii Telescope (CFHT) and Subarcsecond Imaging Spectrograph. Lauer and collaborators have shown that NGC 4486B is similar to M31 in having a double nucleus. Here we show that it also resembles M31 in its kinematics. Like M31, NGC 4486B rotates fairly rapidly near the center (V = 76 ± 7 km s at 0. 6) but more slowly farther out (V ≃ 20 ± 6 km s at r ≃ 4). Also, the velocity dispersion gradient is very steep: σ increases from 116 ± 6 km s at r = 2 – 6 to σ = 281± 11 km s at the center. This is much higher than expected for an elliptical galaxy of absolute magnitude MB ≃ −16.8: even more than M31, NGC 4486B is far above the scatter in the Faber-Jackson correlation between σ and bulge luminosity. Therefore the King core mass-to-light ratio, M/LV ≃ 20, is unusually high compared with normal values for old stellar populations (M/LV = 4± 1 at MB ≃ −17). We construct simple dynamical models with isotropic velocity dispersions and show that they reproduce black hole (BH) masses derived by more detailed methods. We also fit axisymmetric, three-integral models. Isotropic models imply that NGC 4486B contains a central dark object, probably a BH, of mass M• = 6 +3 −2 × 10 M⊙. However, anisotropic models fit the data without a BH if the ratio of radial to azimuthal dispersions is ∼ 2 at r ≃ 1. Therefore this is a less strong BH detection than the ones in M31, M32, and NGC 3115. A dark mass of 6 × 10M⊙ is ∼ 9% of the mass Mbulge in stars; even if M• is somewhat smaller than the isotropic value, M•/Mbulge is likely to be unusually large. Double nuclei are a puzzle because the dynamical friction timescales for self-gravitating star clusters in close orbit around each other are short. Since both M31 and NGC 4486B contain central dark objects, our results support models in which the survival of a double nucleus is connected with the presence of a BH. For example, they support the Keplerian eccentric disk model due to Tremaine. Subject headings: black hole physics – galaxies: individual (NGC 4486B) – galaxies: kinematics and dynamics – galaxies: nuclei