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Abstract: We show that the mass-segregation solution for the steady state distributionof stars around a massive black hole MBH has two branches: the known weaksegregation solution Bahcall and Wolf 1977, and a newly discovered strongsegregation solution, presented here. The nature of the solution depends on theheavy-to-light stellar mass ratio M H-M L and on the unbound population numberratio N H-N L, through the relaxational coupling parameter \Delta=4 N H M H^2-N L M L^23+M H-M L. When the heavy stars are relatively common\Delta>>1, they scatter frequently on each other. This efficientself-coupling leads to weak mass segregation, where the stars form n \proptor^{-\alpha M} mass-dependent cusps near the MBH, with indices \alpha H=7-4 forthe heavy stars and 3-2<\alpha L<7-4 for the light stars i.e.\max\alpha H-\alpha L~=1-4. However, when the heavy stars are relativelyrare \Delta<<1, they scatter mostly on light stars, sink to the center bydynamical friction and settle into a much steeper cusp with 2~<\alpha H<11-4,while the light stars form a 3-2<\alpha L<7-4 cusp, resulting in strongsegregation i.e. \max\alpha H-\alpha L~=1. We show that the present-daymass function of evolved stellar populations coeval or continuously starforming with a universal initial mass function, separate into two distinctmass scales, ~1 Mo of main sequence and compact dwarfs, and ~10 Mo of stellarblack holes SBHs, and have \Delta<0.1. We conclude that it is likely thatmany relaxed galactic nuclei are strongly segregated. We review indications ofstrong segregation in observations of the Galactic Center and in results ofnumeric simulations, and briefly list some possible implications of a very highcentral concentration of SBHs around a MBH.



Author: Tal Alexander Weizmann Inst., Clovis Hopman Leiden Obs.

Source: https://arxiv.org/







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