Eccentric-Disk Models for the Nucleus of M31

We construct dynamical models of the "double" nucleus of M31 in which the nucleus consists of an eccentric disk of stars orbiting a central black hole. The principal approximation in these models is that the disk stars travel in a Keplerian potential; i.e., we neglect the mass of the disk relative to the black hole. We consider both "aligned" models, in which the eccentric disk lies in the plane of the large-scale M31 disk, and "nonaligned" models, in which the orientation of the eccentric disk is fitted to the data. Both types of model can reproduce the double structure and overall morphology seen in Hubble Space Telescope photometry. In comparison with the best available ground-based spectroscopy, the models reproduce the asymmetric rotation curve, the peak height of the dispersion profile, and the qualitative behavior of the Gauss-Hermite coefficients h₃ and h₄. Aligned models fail to reproduce the observation that the surface brightness at P1 is higher than at P2 and yield significantly poorer fits to the kinematics; thus, we favor nonaligned models. Eccentric-disk models fitted to ground-based spectroscopy are used to predict the kinematics observed at much higher resolution by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, and we find generally satisfactory agreement.