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Free keywords:
Astrophysics, Galaxy Astrophysics, astro-ph.GA,Astrophysics, Cosmology and Extragalactic Astrophysics, astro-ph.CO,General Relativity and Quantum Cosmology, gr-qc
Abstract:
We present, for the first time, a clear $N$-body realization of the {\it
strong mass segregation} solution for the stellar distribution around a massive
black hole. We compare our $N$-body results with those obtained by solving the
orbit-averaged Fokker-Planck (FP) equation in energy space. The $N$-body
segregation is slightly stronger than in the FP solution, but both confirm the
{\it robustness} of the regime of strong segregation when the number fraction
of heavy stars is a (realistically) small fraction of the total population. In
view of recent observations revealing a dearth of giant stars in the sub-parsec
region of the Milky Way, we show that the time scales associated with cusp
re-growth are not longer than $(0.1-0.25) \times T_{rlx}(r_h)$. These time
scales are shorter than a Hubble time for black holes masses $\mbul \lesssim 4
\times 10^6 M_\odot$ and we conclude that quasi-steady, mass segregated,
stellar cusps may be common around MBHs in this mass range. Since EMRI rates
scale as $\mbul^{-\alpha}$, with $\alpha \in [1\4,1]$, a good fraction of these
events should originate from strongly segregated stellar cusps.
