Dynamics in galaxy centers

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Astronomers now believe that the centers of most galaxies harbor extremely large black holes, containing millions to billions of times the mass of our Sun. These black holes can be detected only through their influence on their neighbors, including stars, dust, and gas. The only way to study this influence, and especially to interpret the astronomical observations, is through complex computer simulations. This project will develop several new approaches to understanding the behavior of stars near black holes, emphasizing the observable consequences. Much of the detailed work will be carried out by junior scientists who will thus undergo extensive advanced training in theoretical astrophysics.Particular effects of importance include flares from stars that wander too close to the black hole, from active galactic nuclei fed by gaseous accretion disks orbiting the black hole, and from the influence of the black hole's gravity on the kinematics of nearby stars. As noted, this research will develop novel numerical tools to study the dynamics of stellar systems near black holes, and in addition will study the evolution of warps in accretion disks and look for relativistic effects in the spectra of quasars. Specifically, this work will: (i) explore novel orbit integrators for the gravitational N-body problem, possibly allowing much larger time-steps; (ii) investigate the behavior of warped accretion disks, where self-gravity, viscous stresses, and the Lense-Thirring effect, are all important in determining the disk shape; (iii) develop an "N-wire" code (which averages over orbital phase by representing each star as an eccentric wire) and use it to study relaxation and stability of stellar systems near a black hole and the eccentric stellar disk at the center of M31; (iv) implement an "N-ring" code (which represents each star as an annular disk) and use it to model stars near the Galactic center; and (v) look for relativistic effects in the velocity offsets between broad and narrow emission lines in a large sample of quasars. The project will contribute to the advanced training of graduate students and postdoctoral fellows in theoretical astrophysics, and will help to interpret results from existing and planned flagship telescope projects.