STRONG MASS SEGREGATION AROUND A MASSIVE BLACK HOLE
By T. Alexander, Clovis Hopman. 2008
137 citations recorded by the source corpus.
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 = 4NHM2H/[NLM2L(3 + MH/ML)]. When the heavy stars are relatively common (Δ ≫ 1), they scatter frequently on each other. This efficient self-coupling leads to weak mass segregation, where the stars form mass-dependent cusps near the MBH, with indices αH = 7/4 for the heavy stars and 3/2 < αL < 7/4 for the light stars (i.e. max(αH − αL) ≃ 1/4). However, when the heavy stars are relatively rare (Δ ≪ 1), they scatter mostly on light stars, sink to the center by dynamical friction and settle into a much steeper cusp with 2 ≲ αH ≲ 11/4, while the light stars form a 3/2 < αL < 7/4 cusp, resulting in strong segregation (i.e., max(αH − αL) ≃ 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☉ of main sequence and compact dwarfs, and ∼10 M☉ 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.
Subfields: Astronomy and Astrophysics, Instrumentation
View Paper: https://www.semanticscholar.org/paper/d9dbe56c6d53cebe421e9df9775d5afd04f8465c
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Citations in the corpus
- Eccentric Black Hole Gravitational-wave Capture Sources in Galactic Nuclei: Distribution of Binary Parameters (2017, methodology): “In this multi-mass BH population model, we assume a power-law PMF b-mBH following Alexander & Hopman (2009) for 10 Gyr of continuous star formation.”
- Black Hole Mergers in Galactic Nuclei Induced by the Eccentric Kozai–Lidov Effect (2017, methodology): “However, if the density profile of the stellar BHs is steeper, like the one that might be expected from mass segregation over long timescales (e.g. Keshet et al. 2009), the evaporation timescale might be shorter.”
- Stellar Dynamics and Stellar Phenomena Near a Massive Black Hole (2017, background): “…I• ∼ O(I?). www.annualreviews.org • Stars near massive black holes 13 The effects of strong mass segregation on the density distribution of a simplified population with MS stars and remnants (WDs, NSs and stellar BHs) around a Milky Way-like nucleus with M• = 4× 106 M (Alexander & Hopman 2009).”
- Rapid and Bright Stellar-mass Binary Black Hole Mergers in Active Galactic Nuclei (2016, methodology): “…eccentricity of the CoM of the binary is drawn from an isotropic thermal distribution with f (e) = 12 e and a strongly mass segregated number density n(a) ∝ a−2.5, which is also consistent with the massive O star distribution in the Galactic Center (Bartko et al. 2009; Alexander & Hopman 2009).”
- Merging Binaries in the Galactic Center: The eccentric Kozai-Lidov mechanism with stellar evolution (2016, background): “…within the central 1 pc play a significant role in numerous processes that take place at the GC, including the relaxation state of the GC (Alexander & Hopman 2009)1, the stellar number density (e.g., Alexander & Pfuhl 2014; Prodan et al. 2015), the S-star cluster population (e.g.,…”