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Two stellar-mass black holes in the globular cluster M22


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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Figure 1: VLA radio continuum image of the core of the globular cluster M22.
Figure 2: Optical images of M22 and the candidate companion stars to the radio sources.
Figure 3: Radio–X-ray correlation for stellar-mass black holes.


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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).

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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.

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Correspondence to Jay Strader.

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The authors declare no competing financial interests.

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Strader, J., Chomiuk, L., Maccarone, T. et al. Two stellar-mass black holes in the globular cluster M22. Nature 490, 71–73 (2012).

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