Self-Gravitating Eccentric Disk Models for the Double Nucleus of M31

Abstract

We present new dynamical models of weakly self-gravitating, finite dispersion eccentric stellar disks around central black holes for the double nucleus of M31. The disk is fixed in a frame rotating at constant precession speed, and is populated by stars on quasi-periodic orbits whose parents are numerically integrated periodic orbits in the total potential. A distribution of quasi-periodic orbits about a given parent is approximated by a distribution of Kepler orbits dispersed in eccentricity and orientation, using an approximate phase-space distribution function written in terms of the integrals of motion in the Kepler problem. We use these models, along with an optimization routine, to fit available published kinematics and photometry in the inner 2 arcseconds of the nucleus. A grid of 24 best-fit models is computed to accurately constrain the mass of the central black hole and nuclear disk parameters. We find that the supermassive black hole in M31 has mass $M_{BH} = 5.62 \pm 0.66 \times 10^7 \msun$, which is consistent with the observed correlation between the central black hole mass and the velocity dispersion of its host spheroid. Our models precess rapidly, at $Ω= 36.5 \pm 4.2 \kmspc$, and possess a characteristic radial eccentricity distribution, which gives rise to multi-modal line of sight velocity distributions along lines of sight near the black hole. These features can be used as sensitive discriminants of disk structure.