On the distribution of stellar remnants around massive black holes: slow   mass segregation, star cluster inspirals and correlated orbits
  We study the long term dynamical evolution of stellar mass black holes (BHs) at the Galactic center (GC) and put constraints on their number and central mass distribution. Models of the GC are considered that have not yet achieved a steady state under the influence of random gravitational encounters. Contrary to some recent claims that mass-segregation can rapidly rebuild a density cusp in the stars, we find that time scales associated with cusp regrowth are longer than the Hubble time. These results cast doubts on standard models that postulate high densities of BHs near the GC and motivate studies that start from initial conditions which correspond to well-defined physical models. For the first time, we consider the distribution of BHs in a dissipationless formation model for the Milky Way nuclear cluster (NC), in which massive stellar clusters merge in the GC to form a nucleus. We simulate the successive inspiral of massive clusters containing an inner dense cluster of BHs. The pre-existing mass segregation is not completely erased as the clusters are disrupted by the massive black hole tidal field. As a result, after 12 inspiral events a NC forms in which the BHs have higher central densities than the stars. After evolving the model for 5-10 Gyr, the BHs do form a steep central cusp, while the stellar distribution maintains properties that resemble those of the Milky Way NC. Finally, we investigate the effect of BH perturbations on the motion of the GC S-stars, as a means of constraining the number of the perturbers. We find that reproducing the S-star orbital distribution requires >~1000 BHs within 0.1 pc of Sgr A*. A dissipationless formation scenario for the Milky Way NC is consistent with this lower limit and therefore could reconcile the need for high central densities of BHs (to explain the orbits of the S-stars), with the missing-cusp problem of the GC giant star population. 
Fabio Antonini (CITA)
astro-ph.GA