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Z.I. Tsvetanov (JHU), Y.C. Pei (STScI), H. C. Ford (JHU), G.A. Kriss (STScI), R.J. Harms (RJH Sci.)
We have used the Faint Object Spectrograph on HST with its 0\farcs26 and 0\farcs09 apertures to sample the stellar rotational velocities and velocity dispersions in the nucleus of M31 along the line connecting the two surface brightness peaks separated by 0\farcs48. We discover a strong asymmetry in the rotation curve which reaches a maximum of -270 km~s-1 on the side of the faint peak (P2) and +160 km~s-1 on the side of the bright peak (P1). The center of rotation lies in between the two peaks. The observed velocity dispersion reaches a maximum of ~300 km~s-1 at the location of P2 and remains flat at ~150 km~s-1 across P1. Our kinematic data strongly support the hypothesis, suggested by Tremaine, that the nucleus of M31 is a thick stellar eccentric disk orbiting around a massive black hole located at P2. The bright peak is a result of crowding of stars due to slowing at the apoapsis of the eccentric disk. With simple exponential profiles for the disk model, we can fit nearly all of the photometric and kinematic data, yielding an estimate of mass of (7.0±.4)\times107 M\odot for the black hole in the nucleus of M31.
Apart from velocity shifts and line broadening, the FOS spectra of P1 and the region immediately around P2 differ only by a weak blue component that is most likely light from the extended UV-bright source in P2 yielding further support for the eccentric disk model. The spectrum of P1 plus a model star with T\rm eff = 7000 K is a good match to P2 (better than P1 plus a power law). This result is consistent with Lauer et al.\ (1998) conclusion that the black hole in P2 is surrounded by a small cluster of late-B to early-A stars, analogous to the star cluster surrounding the black hole in the center of the Milky Way.