STRONG MASS SEGREGATION AROUND A MASSIVE BLACK HOLE

被引:180
作者
Alexander, Tal [1 ]
Hopman, Clovis [2 ]
机构
[1] Weizmann Inst Sci, Fac Phys, IL-76100 Rehovot, Israel
[2] Leiden Univ, Leiden Observ, NL-2300 RA Leiden, Netherlands
基金
欧洲研究理事会; 芬兰科学院;
关键词
black hole physics; Galaxy: kinematics and dynamics; stellar dynamics; GRAVITATIONAL-WAVE SOURCES; ACTIVE GALACTIC NUCLEI; YOUNG STAR-CLUSTERS; STELLAR ORBITS; SGR-A; RUNAWAY COLLISIONS; GLOBULAR-CLUSTERS; COMPACT REMNANTS; TIDAL DISRUPTION; COLLAPSED OBJECT;
D O I
10.1088/0004-637X/697/2/1861
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
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 M(H)/M(L) and on the unbound population number ratio N(H)/N(L), through the relaxational coupling parameter Delta = 4N(H)M(H)(2)/[N(L)M(L)(2)(3 + M(H)/M(L))]. When the heavy stars are relatively common (Delta >> 1), they scatter frequently on each other. This efficient self-coupling leads to weak mass segregation, where the stars form n alpha r(-alpha M) mass-dependent cusps near the MBH, with indices alpha(H) = 7/4 for the heavy stars and 3/2 < alpha(L) < 7/4 for the light stars (i.e. max(alpha(H) - alpha(L)) similar or equal to 1/4). However, when the heavy stars are relatively rare (Delta << 1), they scatter mostly on light stars, sink to the center by dynamical friction and settle into a much steeper cusp with 2 less than or similar to alpha(H) less than or similar to 11/4, while the light stars form a 3/2 < alpha(L) < 7/4 cusp, resulting in strong segregation (i.e., max(alpha(H) - alpha(L)) similar or equal to 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, similar to 1 M(circle dot) of main sequence and compact dwarfs, and similar to 10 M(circle dot) of stellar black holes (SBHs), and have Delta < 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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页码:1861 / 1869
页数:9
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