2010/02/05 by Clovis Hopman, Hopman, Clovis, Ann-Marie Madigan +1
Physics and Astronomy · #Cosmology and Gravitation Theories #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies #astro-ph.GA
paper · pdf · doi:10.48550/arxiv.1002.1220
Proceedings article to be published in "The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies", ed. Mark Morris, Daniel Q. Wang and Feng Yuan
arxiv created 2010/02/05 · arxiv updated 2010/02/26
Two-body energy exchange between stars orbiting massive black holes (MBHs) leads to the formation of a power-law density distribution n(r)~r^(-a) that diverges towards the MBH. For a single mass population, a=7/4 and the flow of stars is much less than N(<r)/tr (enclosed number of stars per relaxation time). This "zero-flow" solution is maintained for a multi-mass system for moderate mass ratios or systems where there are many heavy stars, and slopes of 3/2<a<2 are reached, with steeper slopes for the more massive stars. If the heavy stars are rare and massive however, the zero-flow limit breaks down and much steeper distributions are obtained. We discuss the physics driving mass-segregation with the use of Fokker-Planck calculations, and show that steady state is reached in 0.2-0.3 tr. Since the relaxation time in the Galactic centre (GC) is tr ~2-3 * 10^(10) yr, a cusp should form in less than a Hubble time. The absence of a visible cusp of old stars in the GC poses a challenge to these models, suggesting that processes other than two-body relaxation have played a role. We discuss astrophysical processes within the GC that depend crucially on the details of the stellar cusp.