Phase Mixing in Stellar Systems and the Nucleus of M31

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AST-9900316Phase mixing in stellar systems and the nucleus of M31Scott TremaineDr. Tremaine is studying two aspects of the dynamics of systems of stars such as galaxies and star clusters. The first is phase mixing, the process by which a stellar system of collisionless plasma reaches equilibrium. Phase mixing is a gradual process, in which irregularities present at the formation of the system become more and more tightly wrapped over many orbits so that macroscopically the stellar system becomes smoother and smoother. The persistence of these irregularities leads to several important effects that are investigated: gravitational coupling between the disk and dark halo in the inner parts of galaxies, that may spin up and flatten the inner halo; enhanced relaxation near the centers of galaxies that may isotropize the stellar distribution function; and the presence of prominent unbound structures in the phase space distribution of stars. The second is the dynamics of eccentric stellar disks such as the one believed to be present in the center of the nearby galaxy M31. Such disks can arise naturally in a nearly Keplerian gravitational potential, for example close to the massive central black hole that is believed to sit in the centers of nearby galaxies. Dr. Tremaine is studying the stability and normal modes of axisymmetric stellar disks in nearly Keplerian potentials, to determine whether eccentric stellar disks arise naturally and persist for the age of the galaxy.