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 sigma* = 0.'' 22 with the Canada-France-Hawaii Telescope 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(-1) at 0.'' 6) but more slowly farther out (V similar or equal to 20 +/- 6 km s(-1) at r similar or equal to 4 ''). Also, the velocity dispersion gradient is very steep: a increases from 116 +/- 6 km s(-1) at r = 2 ''-6 '' to sigma = 281 +/- 11 km s(-1) at the center. This is much higher than expected for an elliptical galaxy of absolute magnitude M-B similar or equal to -16.8: even more than M31, NGC 4486B is far above the scatter in the Faber-Jackson correlation between a and bulge luminosity. Therefore, the King core mass-to-light ratio, M/L-V similar or equal to 20, is unusually high compared with normal values for old stellar populations (M/L-V = 4 +/- 1 at M-B similar or equal to -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(-2)(+3) x 10(8) M.. However, anisotropic models fit the data without a BH if the ratio of radial to azimuthal dispersions is similar to 2 at r similar or equal to 1 ''. Therefore, this is a less strong BH detection than the ones in M31, M32, and NGC 3115. A dark mass of 6 x 10(8) M. is similar to 9% of the mass M-bulge in stars; even if M. is somewhat smaller than the isotropic value, M./M-bulge 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.