The Distribution of Stars and Stellar Remnants at the Galactic Center

Motivated by recent observations that suggest a low density of old stars around the Milky Way supermassive black hole (SMBH), models for the nuclear star cluster are considered that have not yet reached a steady state under the influence of gravitational encounters. A core of initial radius 1–1.5 pc evolves to a size of approximately 0.5 pc after 10 Gyr, roughly the size of the observed core. The absence of a Bahcall–Wolf cusp is naturally explained in these models, without the need for fine-tuning or implausible initial conditions. In the absence of a cusp, the time for a 10 M ☉ black hole (BH) to spiral in to the Galactic center from an initial distance of 5 pc can be much greater than 10 Gyr. Assuming that the stellar BHs had the same phase-space distribution initially as the stars, their density after 5–10 Gyr is predicted to rise very steeply going into the stellar core, but could remain substantially below the densities inferred from steady-state models that include a steep density cusp in the stars. Possible mechanisms for the creation of the parsec-scale initial core include destruction of stars on centrophilic orbits in a pre-existing triaxial nucleus, inhibited star formation near the SMBH, or ejection of stars by a massive binary. The implications of these models are discussed for the rates of gravitational-wave inspiral events, as well as other physical processes that depend on a high density of stars or stellar-mass BHs near SgrA*.