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Session 111 - Massive Black Holes and Dynamics of Galaxy Cores.
Oral session, Thursday, January 16
\newdimenßss \def\sdßss=.1em \ifmmode \rlap.''\kern -ßss \else \rlap.''\kern -ßss\fi The stellar kinematics of the low-luminosity elliptical galaxy NGC 4486B have been measured in seeing FWHM = 0\sd66 with the Canada-France-Hawaii Telescope. Lauer et al. (1996, ApJL, in press) have shown that NGC 4486B is similar to M\thinspace31 in having a double nucleus. We show that it also resembles M\thinspace31 in its dynamics. The region near the double nucleus rotates more rapidly than the rest of the galaxy and has a steep velocity dispersion gradient. The central dispersion, \sigma = 257 \pm 9 km s^-1, is much higher than expected for an elliptical galaxy of absolute magnitude M_B \simeq -16.8 (distance = 16 Mpc). Even more than M\thinspace31, NGC 4486B is far above the scatter in the Faber-Jackson correlation between \sigma and bulge luminosity. Given the observed brightness distribution, this implies that the central mass-to-light ratio is unusually high. We construct dynamical models with isotropic velocity dispersions; these imply that NGC 4486B contains a central dark object, probably a black hole (BH), of mass M_\bullet \simeq (5.3^+3.0_-2.5) \times 10^8 M_ødot. Smaller masses are allowed if the velocity distribution is anisotropic, but M_\bullet is likely to be _>\atop^\sim 10^8 M_ødot. This BH detection reinforces the observed correlation of M_\bullet with bulge luminosity (Kormendy amp; Richstone 1995, ARAamp;A, 33, 581). We now know of two nearby galaxies with double nuclei; this increases the need for an explanation that allows the double structure to last longer than the (short!) dynamical friction timescale of an accretion. The detection of a central dark object in NGC 4486B supports models in which double nuclei depend on the presence of a BH (e.\thinspaceg., Tremaine 1995, AJ, 110, 628).
JK's work was supported by NSF grant AST-9219221. RB was supported by SFB 375 and by the Max-Planck-Gesellschaft. The Nuker team was supported by HST data analysis funds through grant GO-02600.01-87A and by NSERC.
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