Formation of massive black holes through runaway collisions in dense young star clusters

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Abstract

A luminous X-ray source is associated with MGG 11—a cluster of young stars 200 pc from the centre of the starburst galaxy M 82 (refs 1, 2). The properties of this source are best explained3,4 by invoking a black hole with a mass of at least 350 solar masses (350 M), which is intermediate between stellar-mass and supermassive black holes. A nearby but somewhat more massive cluster (MGG 9) shows no evidence of such an intermediate-mass black hole1,3, raising the issue of just what physical characteristics of the clusters can account for this difference. Here we report numerical simulations of the evolution and motion of stars within the clusters, where stars are allowed to merge with each other. We find that for MGG 11 dynamical friction leads to the massive stars sinking rapidly to the centre of the cluster, where they participate in a runaway collision. This produces a star of 800–3,000 M, which ultimately collapses to a black hole of intermediate mass. No such runaway occurs in the cluster MGG 9, because the larger cluster radius leads to a mass segregation timescale a factor of five longer than for MGG 11.

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Figure 1: Chandra X-ray image of the relevant part of the starburst galaxy M 82 with the observed star clusters indicated.
Figure 2: The growth in mass of the collision runaway star with time.
Figure 3: The area of parameter space for which runaway collision can occur, and where the process is prevented.

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Acknowledgements

We thank N. McCrady and D. Pooley for discussions on MGG 11, T. G. Tsuru for an accurate position of M82 X-1, and E. van den Heuvel for critically reading the manuscript. This work was supported by NASA ATP, the Royal Netherlands Academy of Sciences (KNAW), the Dutch Organization of Science (NWO), and the Netherlands Research School for Astronomy (NOVA).

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Correspondence to Simon F. Portegies Zwart.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Information

Includes discussion of the selection of initial conditions, numerical methods, and the results of simulations of the star clusters MGG-9 and MGG-11; Supplementary Figure 1: Evolution of the core radius for four simulations of the star cluster MGG-11; Supplementary Figure 2: Stellar radius as a function of zero-age mass for various models of high-mass stars. (PDF 196 kb)

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Portegies Zwart, S., Baumgardt, H., Hut, P. et al. Formation of massive black holes through runaway collisions in dense young star clusters. Nature 428, 724–726 (2004) doi:10.1038/nature02448

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