Hundreds of stellar-mass black holes likely form in a typical globular starcluster, with all but one predicted to be ejected through dynamicalinteractions. Some observational support for this idea is provided by the lackof 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 manyneutron-star/X-ray binaries are known. Although a few black holes have beenseen in globular clusters around other galaxies, the masses of these cannot bedetermined, and some may be intermediate-mass black holes that form throughexotic mechanisms. Here we report the presence of two flat-spectrum radiosources in the Milky Way globular cluster M22, and we argue that these objectsare 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 forthese black holes, consistent with the larger predicted masses of black holesin globular clusters compared to those outside. The identification of two blackholes in one cluster shows that the ejection of black holes is not as efficientas predicted by most models, and we argue that M22 may contain a totalpopulation of ~ 5-100 black holes. The large core radius of M22 could arisefrom heating produced by the black holes.
展开▼
