ABSTRACT

We have measured the line-of-sight velocity profiles of M32. The major axis velocity profiles are asymmetric, with opposite asymmetry on opposite sides of the nucleus. Existing models for M32 cannot account for these asymmetries. We present new models which assume the distribution function to be of the form f = f(E, Lz). Such models require a central black hole of ∼ 1.8 × 106 M⊙ to fit the observed rotation velocities and velocity dispersions. Without invoking any further free parameters, these models provide a good fit to the observed velocity profile asymmetries.

OBSERVED VELOCITY PROFILES

The presence of a massive black hole has been invoked to match the observed rotation velocities and velocity dispersions at the center of M32 (Tonry 1987; Richstone, Bower and Dressier 1990). Previous studies have assumed the line-of-sight velocity distributions of the stars, henceforth referred to as the velocity profiles, to be Gaussian. We have determined the velocity profile shapes of M32 from high S/N spectra taken with the William Herschel Telescope at La Palma (van der Marel et al. 1993), using the techniques of Rix and White (1992) and van der Marel and Franx (1993). The velocity profiles are asymmetric, with the asymmetry changing sign upon going from one side of the nucleus to the other (see Fig. 1). None of the existing models, in which the local (unprojected) velocity distributions of the stars are assumed to be Gaussian, can reproduce the observed asymmetries of the velocity profiles.