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_* = 022 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 06) but more slowly farther out (V 20 ± 6 km s^-1 at r 4''). Also, the velocity dispersion gradient is very steep: σ increases from 116 ± 6 km s^-1 at r = 2''-6'' to σ = 281 ± 11 km s^-1 at the center. This is much higher than expected for an elliptical galaxy of absolute magnitude M_B -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/L_V 20, is unusually high compared with normal values for old stellar populations (M/L_V = 4 ± 1 at M_B -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 × 10⁸ 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 × 10⁸ M_☉ is ~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.