Letter | Published:

Two stellar-mass black holes in the globular cluster M22

Nature volume 490, pages 7173 (04 October 2012) | Download Citation


Hundreds of stellar-mass black holes probably form in a typical globular star cluster, with all but one predicted to be ejected through dynamical interactions1,2,3. Some observational support for this idea is provided by the lack of X-ray-emitting binary stars comprising one black hole and one other star (‘black-hole/X-ray binaries’) in Milky Way globular clusters, even though many neutron-star/X-ray binaries are known4. Although a few black holes have been seen in globular clusters around other galaxies5,6, the masses of these cannot be determined, and some may be intermediate-mass black holes that form through exotic mechanisms7. Here we report the presence of two flat-spectrum radio sources in the Milky Way globular cluster M22, and we argue that these objects are black holes of stellar mass (each 10–20 times more massive than the Sun) that are accreting matter. We find a high ratio of radio-to-X-ray flux for these black holes, consistent with the larger predicted masses of black holes in globular clusters compared to those outside8. The identification of two black holes in one cluster shows that ejection of black holes is not as efficient as predicted by most models1,2,4, and we argue that M22 may contain a total population of 5–100 black holes. The large core radius of M22 could arise from heating produced by the black holes9.

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The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. L.C. is a Jansky Fellow of the National Radio Astronomy Observatory. This work is partially based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).

Author information


  1. Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA

    • Jay Strader
    •  & Laura Chomiuk
  2. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA

    • Jay Strader
    •  & Laura Chomiuk
  3. National Radio Astronomy Observatory, PO Box O, Socorro, New Mexico 87801, USA

    • Laura Chomiuk
  4. School of Physics and Astronomy, University of Southampton, Highfield SO17 IBJ, UK

    • Thomas J. Maccarone
  5. International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia

    • James C. A. Miller-Jones
  6. Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA

    • Anil C. Seth


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J.S. wrote the text. L.C. reduced the data. All authors contributed to the interpretation of the data and commented on the final manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Jay Strader.

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