On the Nuclear Rotation Curve of M31

The nuclear rotation curve of M31, as observed by the Hubble Space TelescopeFaint Object Camera Spectrograph, shows a significant disturbance coincidingwith the off-center brightness peak, P1. This +/- 60 km/s feature isdistinguished by a local velocity maximum centered on P1 and a local minimumapproxmately 0.08" closer to P2. If the M31 double nucleus is an eccentric diskwith an off-center density concentration, as suggested by Tremaine, then theself-gravity of the disk can produce just such a disturbance. The expectedkinematic signature is calculated approximately by examining sequences ofclosed periodic orbits in a Kepler potential perturbed by a model diskpotential that precesses at constant frequency. The perturbation forces a steepnegative eccentricity gradient in the sequence of closed orbits through thedensest part of the disk, which reverses the arrangement of periapsis andapoapsis with respect to the central mass. Stars making up the inner part ofthe density concentration are at apoapsis, while stars making up the outer partare at periapsis, producing a steep local velocity gradient. This result isindependent of the details of the mass distribution. The projected rotationcurve of the model is shown to closely resemble that of M31, giving strongsupport to the eccentric disk picture.Comment: 10 pages, 4 eps figures, LaTeX, AASTeX 4.0. Submitted to ApJ Letter