Spectroscopic Evidence for a Supermassive Black Hole in NGC 4486B

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Abstract
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 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 × 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 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.
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