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

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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.


  1. 1

    Matsumoto, H. & Tsuru, T. G. X-ray evidence of an AGN in M82. Publ. Astron. Soc. Jpn 51, 321–331 (1999); Erratum. Publ. Astron. Soc. Jpn. 51, 567 (1999)

  2. 2

    Kaaret, P. et al. Chandra high-resolution camera observations of the luminous X-ray source in the starburst galaxy M82. Mon. Not. R. Astron. Soc. 321, L29–L32 (2001)

  3. 3

    Matsumoto, H. et al. Discovery of a luminous, variable, off-center source in the nucleus of M82 with the Chandra high-resolution camera. Astrophys. J. 547, L25–L28 (2001)

  4. 4

    Strohmayer, T. E. & Mushotsky, R. F. Discovery of X-ray quasi-periodic oscillations from an ultraluminous X-ray source in M82: Evidence against beaming. Astrophys. J. 586, L61–L64 (2003)

  5. 5

    McCrady, N., Gilbert, A. M. & Graham, J. R. Kinematic masses of super star clusters in M82 from high-resolution near-infrared spectroscopy. Astrophys. J. 596, 240–252 (2003)

  6. 6

    Portegies Zwart, S. F., Makino, J., McMillan, S. L. W. & Hut, P. Star cluster ecology. III. Runaway collisions in young compact star clusters. Astron. Astrophys. 348, 117–126 (1999)

  7. 7

    Portegies Zwart, S. F. & McMillan, S. L. W. The runaway growth of intermediate-mass black holes in dense star clusters. Astrophys. J. 576, 899–907 (2002)

  8. 8

    Gürkan, M. A., Freitag, M. & Rasio, F. A. Formation of massive black holes in dense star clusters. I. Mass segregation and core collapse. Astrophys. J. (in the press); preprint at 〈〉 (2003)

  9. 9

    Ebisuzaki, T. et al. Missing link found? The “runaway” path to supermassive black holes. Astrophys. J. 562, L19–L22 (2001)

  10. 10

    Spitzer, L. Jr & Hart, M. H. Random gravitational encounters and the evolution of spherical systems. I. Method. Astrophys. J. 164, 399–409 (1971)

  11. 11

    Portegies Zwart, S. F., McMillan, S. L. W., Hut, P. & Makino, J. Star cluster ecology-IV. Dissection of an open star cluster: photometry. Mon. Not. R. Astron. Soc. 321, 199–226 (2001)

  12. 12

    Hut, P., Makino, J., McMillan, S. L. W. & Portegies Zwart, S. Starlab〉 (1995).

  13. 13

    Aarseth, S. J. From NBODY1 to NBODY6: The growth of an industry. Publ. Astron. Soc. Pacif. 111, 1333–1346 (1999)

  14. 14

    Baumgardt, H. & Makino, J. Dynamical evolution of star clusters in tidal fields. Mon. Not. R. Astron. Soc. 340, 227–246 (2003)

  15. 15

    King, I. R. The structure of star clusters. III. Some simple dynamical models. Astron. J. 71, 64–75 (1966)

  16. 16

    Heger, A., Fryer, C. L., Woosley, S. E., Langer, N. & Hartmann, D. H. How massive single stars end their life. Astrophys. J. 591, 288–300 (2003)

  17. 17

    Stothers, B. & Chin, C.-W. On two theories of the cyclical outbursts of eta Carinae. Astrophys. J 489, 319–330 (1997)

  18. 18

    Ishii, M., Munetaka, U. & Kato, M. Core-halo structure of a chemically homogeneous massive star and bending of the zero-age main sequence. Publ. Astron. Soc. Jpn 51, 417–424 (1999)

  19. 19

    Hopman, C., Portegies Zwart, S. F. & Alexander, T. Ultraluminous X-ray sources as intermediate mass black holes fed by tidally captured stars. Astrophys. J. Lett. (submitted); preprint at 〈〉 (2003)

  20. 20

    Heggie, D. C. & Hut, P. The Gravitational Million-Body Problem (Cambridge Univ. Press, Cambridge, 2003)

  21. 21

    Whitmore, B. C. et al. The luminosity function of young star clusters in “The Antennae” galaxies. Astrophys. J. 118, 1551–1576 (1999)

  22. 22

    Mengel, S., Lehnert, M. D., Thatte, N. & Genzel, R. Dynamical masses of young star clusters in NGC 4038/4039. Astron. Astrophys. 383, 137–152 (2002)

  23. 23

    Zezas, A., Fabbiano, G., Rots, A. H. & Murray, S. S. Chandra observations of “The Antennae” galaxies (NGC 4038/4039). III. X-ray properties and multiwavelength associations of the X-ray source population. Astrophys. J. 577, 710–725 (2002)

  24. 24

    Piatti, A. E., Bica, E. & Clariá, J. J. Fundamental parameters for the highly reddened young open clusters Westerlund 1 and 2. Astron. Astrophys. Suppl. 127, 423–432 (1998)

  25. 25

    Moffat, A. F. J., Drissen, L. & Shara, M. M. NGC 3603 and its Wolf-Rayet stars: Galactic clone of R136 at the core of 30 Doradus, but without the massive surrounding cluster halo. Astrophys. J. 436, 183–193 (1994)

  26. 26

    Campbell, B. et al. Hubble Space Telescope planetary camera images of R136. Astron. J. 104, 1721–1742 (1992)

  27. 27

    Singer, D., Pugh, H. & Li, W. Supernovae 2004ak, 2004al, and 2004am. IAU Circ. No. 8297 (2004)

  28. 28

    Matsushita, M. S. et al. Formation of a massive black hole at the center of the superbubble in M82. Astrophys. J. 545, L107–L111 (2000)

  29. 29

    Salpeter, E. E. The luminosity function and stellar evolution. Astrophys. J. 121, 161–167 (1955)

  30. 30

    Kroupa, P. On the variation of the initial mass function. Mon. Not. R. Astron. Soc. 322, 231–246 (2001)

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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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