Dynamics in Galactic Centers: Mass Segregation, Gravitational Waves, and Tidal Disruption

The centers of most galaxies harbor supermassive black holes orbited by millions of stars and, likely, thousands of stellar-mass black holes. Gravitational scattering drives mass segregation, causing the heavier black holes to sink toward the center and dominate the dynamics of the innermost region. We show that this concentration of compact objects implies that the space-based gravitational-wave observatory, LISA, could detect thousands of stellar-mass black holes in galactic nuclei. The same dynamical processes predict that massive stars should be overrepresented among tidal disruption events (TDEs). Stellar collisions may deplete the density of ordinary stars near the very center, while the disruption of binaries via the Hills mechanism replenishes it. We compare these predictions with observations of the Galactic Center and with recent detections of quasi-periodic X-ray eruptions (QPEs) associated with some tidal disruption events.