Dynamical Modeling of the Stellar Nucleus of M31

Abstract

We present stellar dynamical models of the lopsided, double-peaked nucleus of M31, derived from Hubble Space Telescope (HST) photometry. A Schwarzscild-type method, in conjunction with Richardson-Lucy deconvolution, was employed to construct steadily rotating, hot, stellar disks. The stars orbit a massive dark object, on prograde and retrograde quasi-periodic loop orbits. Our results support Tremaine's eccentric disk model, extended to include a more massive disk, non zero pattern speed ($Ω$), and different viewing angle. Most of the disk mass populated prograde orbits, with $\simeq 3.4%$ on retrograde orbits. The best fits to photometric and kinematic maps were disks with $Ω\approx 16\kmspc $. We speculate on the origins of the lopsidedness, invoking recent work on the linear overstability of nearly Keplerian disks, that possess even a small amount of a counter-rotating component. Accretion of material-no more massive than a globular cluster-onto a preexisting stellar disk, will account for the mass in our retrograde orbits, and could have stimulated the lopsidedness seen in the nucleus of M31.