SPECTROSCOPIC EVIDENCE FOR A SUPERMASSIVE BLACK-HOLE IN NGC 4486B

The stellar kinematics of the low-luminosity elliptical galaxy NGC 448 6B have been measured in seeing sigma = 0.'' 22 with the Canada-Franc e-Hawaii Telescope and Subarcsecond Imaging Spectrograph. Lauer and co llaborators have shown that NGC 4486B is similar to M31 in having a do uble nucleus. Here we show that it also resembles M31 in its kinematic s. 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 equ al to 20 +/- 6 km s(-1) at r similar or equal to 4 ''). Also, the velo city 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. T his 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 a nd bulge luminosity. Therefore, the King core mass-to-light ratio, M/L -V similar or equal to 20, is unusually high compared with normal valu es for old stellar populations (M/L-V = 4 +/- 1 at M-B similar or equa l to -17). We construct simple dynamical models with isotropic velocit y dispersions and show that they reproduce black hole (BH) masses deri ved 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, an isotropic models fit the data without a BH if the ratio of radial to a zimuthal dispersions is similar to 2 at r similar or equal to 1 ''. Th erefore, 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 mas s 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 mo dels in which the survival of a double nucleus is connected with the p resence of a BH. For example, they support the Keplerian eccentric dis k model due to Tremaine.