Two stellar-mass black holes in the globular cluster M22

Two flat-spectrum radio sources in the Milky Way globular cluster M22 are thought to be accreting stellar-mass black holes; the identification of two black holes in one cluster shows that the ejection of black holes from clusters is not as efficient as predicted by most models. One cluster, but two black holes The current consensus is that a typical globular star cluster can accommodate just one stellar-mass black hole. Many more will be created, but all but one will be ejected following dynamical interactions. New observations of the Milky Way globular cluster M22, however, reveal the presence of two radio sources in one cluster, with properties typical of accreting black holes each more than ten times the mass of the Sun. This suggests that the ejection of black holes is not as efficient as predicted by most models. And it may not stop there: the authors speculate that there could be a population of tens of black holes in M22, either as single black holes or in binaries where there is no mass transfer taking place. Hundreds of stellar-mass black holes probably form in a typical globular star cluster, with all but one predicted to be ejected through dynamical interactions 1 , 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 known 4 . Although a few black holes have been seen in globular clusters around other galaxies 5 , 6 , the masses of these cannot be determined, and some may be intermediate-mass black holes that form through exotic mechanisms 7 . 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 outside 8 . The identification of two black holes in one cluster shows that ejection of black holes is not as efficient as predicted by most models 1 , 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 holes 9 .