Spectroscopic Evidence for a Supermassive Black Hole in NGC 4486B

The stellar kinematics of the dwarf elliptical galaxy NGC 4486B have beenmeasured in seeing sigma_* = .22 arcsec with the Canada-France-HawaiiTelescope. Lauer et al. 1996, ApJ, 471, L79 have shown that NGC 4486B issimilar to M31 in having a double nucleus. We show that it also resembles M31in its kinematics. The velocity dispersion gradient is very steep: sigmaincreases from 116 +- 6 km/s at r = 2" - 6" to 281 +- 11 km/s at the center.This is much higher than expected for an elliptical galaxy of absolutemagnitude M_B = -16.8: NGC 4486B is far above the scatter in the Faber-Jacksoncorrelation between sigma and bulge luminosity. Therefore the King coremass-to-light ratio, M/L_V = 20, is unusually high compared with normal valuesfor old stellar populations. We construct dynamical models with isotropicvelocity dispersions and show that they reproduce black hole (BH) massesderived by more detailed methods. We also fit axisymmetric, three-integralmodels. Isotropic models imply that NGC 4486B contains a central dark object,probably a BH, of mass M_BH = 6^{+3}_{-2} x 10^8 M_sun. However, anisotropicmodels fit the data without a BH if the ratio of radial to azimuthaldispersions is ~ 2 at 1". Therefore this is a less strong BH detection than theones in M31, M32, and NGC 3115. A 6 x 10^8 M_sun BH is 9 % of the mass M_bulgein stars; even if M_BH is smaller than the isotropic value, M_BH/M_bulge islikely to be unusually large. Double nuclei are a puzzle because the dynamicalfriction timescales for self-gravitating star clusters in orbit around eachother are short. Since both M31 and NGC 4486B contain central dark objects, ourresults support models in which the survival of double nuclei is connected withthe presence of a BH (e. g., Tremaine 1995, AJ, 110, 628).Comment: 5 pages, 5 figs, TeX, ApJL in pres