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Title:
Strong Mass Segregation Around a Massive Black Hole
Authors:
Alexander, Tal; Hopman, Clovis
Affiliation:
AA(Faculty of Physics, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel ), AB(Leiden University, Leiden Observatory, P.O. box 9513, NL-2300 RA Leiden, Netherlands )
Publication:
The Astrophysical Journal, Volume 697, Issue 2, pp. 1861-1869 (2009). (ApJ Homepage)
Publication Date:
06/2009
Origin:
IOP
ApJ Keywords:
black hole physics, Galaxy: kinematics and dynamics, stellar dynamics
DOI:
10.1088/0004-637X/697/2/1861
Bibliographic Code:
2009ApJ...697.1861A

Abstract

We show that the mass-segregation solution for the steady-state distribution of stars around a massive black hole (MBH) has two branches: the well known weak-segregation solution and a strong segregation solution, which is analyzed here for the first time. The nature of the solution depends on the heavy-to-light stellar mass ratio MH /ML and on the unbound population number ratio NH /NL , through the relaxational coupling parameter Δ = 4NHM 2 H /[NLM 2 L (3 + MH /ML )]. When the heavy stars are relatively common (Δ Gt 1), they scatter frequently on each other. This efficient self-coupling leads to weak mass segregation, where the stars form n\propto r^{-\alpha _{M}} mass-dependent cusps near the MBH, with indices α H = 7/4 for the heavy stars and 3/2 < α L < 7/4 for the light stars (i.e. max(α H - α L ) sime 1/4). However, when the heavy stars are relatively rare (Δ Lt 1), they scatter mostly on light stars, sink to the center by dynamical friction and settle into a much steeper cusp with 2 lsim α H lsim 11/4, while the light stars form a 3/2 < α L < 7/4 cusp, resulting in strong segregation (i.e., max(α H - α L ) sime 1). We show that the present-day mass function of evolved stellar populations with a universal initial mass function (coeval or continuously star forming) separates into two distinct mass scales, ~1 M sun of main sequence and compact dwarfs, and ~10 M sun of stellar black holes (SBHs), and have Δ < 0.1. We conclude that it is likely that many relaxed galactic nuclei are strongly segregated. We review indications of strong segregation in observations of the Galactic center and in results of numeric simulations, and briefly list possible implications of a very high central concentration of SBHs around an MBH.
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