Massive black holes in dense star clusters

Details Massive black holes in dense star clusters Massive black holes in dense star clusters

Nov 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt.........6g&link_type=abstract

Ph.D dissertation, 2005. 162 pages; United States -- Illinois: Northwestern University; 2005. Publication Number: AAT 3177725.

Computer Science

Massive Black Holes, Star Clusters, Black Holes, Runaway Collisions

Scientific paper

In this thesis, we study the formation of massive black holes in dense star clusters and their effects on the clusters' further evolution.
We determine the necessary conditions for early core collapse, leading to the formation of a very massive star via runaway collisions. This process provides a natural mechanism for the formation of intermediate-mass black holes in young dense star clusters, which have been inferred from recent X-ray and optical observations. We performed about a hundred N -body simulations using our Monte Carlo technique, with a wide variety of initial conditions, containing up to 10 7 stars from a broad initial mass distribution. We find that for realistic initial mass functions, mass segregation and dynamical instabilities reduce the core collapse time, t cc , by two orders of magnitude compared to single- component cluster models. The ratio of the core collapse time to central relaxation time is generally ~0.15, which translates into ~0.07 for ratio of core collapse time to half-mass relaxation time for moderately concentrated clusters. This ratio can be smaller if there is initial mass segregation. We also find that typically the mass of the stars in the collapsing core is ~0.2% of the cluster's total mass.
We then study the inspiral towards the Galactic center of young clusters which undergo early core collapse. We find that such clusters can bring many young stars to the central parsec, and hence explain the presence of many young stars in this region. However, this mechanism requires a large initial mass, and deposits more stars, both in the central parsec and outside it, than observed. We provide possible explanations for this discrepancy, and suggest future directions for research.
Finally, we study the effects of primordial binaries on early core collapse runaway collisions. Even though such binaries generally delay and prevent core collapse (e.g., in globular clusters) they are expected to facilitate collisions and hence accelerate early core collapse for systems in which runaway collisions are possible. We report our preliminary results for this work in progress.

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