On the Nuclear Rotation Curve of M31

Abstract

The currently favored model for the double nucleus of M31 consists of an eccentric disk with an off-center density concentration caused by stars lingering at the apocenters of their orbits about the central black hole. If such a disk is to be in equilibrium, the requirement that it precess uniformly because of self-gravity imposes a characteristic radial structure on the orbits that comprise it, creating an observable feature in the rotation curve. The sequence of closed orbits through the densest part of the disk must follow a steep eccentricity gradient, which reverses the arrangement of pericenter and apocenter with respect to the central mass. Stars making up the inner part of the density concentration are at apocenter, while stars making up the outer part are at pericenter, producing a steep local velocity gradient and a distinctive ``wiggle'' in the projected rotation curve near the eccentric density peak. This wiggle closely resembles a feature seen near the off-center brightness peak (P1) in the nuclear rotation curve observed by the Hubble Space Telescope Faint Object Camera Spectrograph. If confirmed by upcoming Space Telescope Imaging Spectrograph observations, this feature will lend further strong support to the eccentric disk picture and provide important constraints for more realistic dynamical models.