Two stellar-mass black holes in the globular cluster M22
  Hundreds of stellar-mass black holes likely form in a typical globular star
cluster, with all but one predicted to be ejected through dynamical
interactions. 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. Although a few black holes have been
seen in globular clusters around other galaxies, the masses of these cannot be
determined, and some may be intermediate-mass black holes that form through
exotic mechanisms. 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. The identification of two black
holes in one cluster shows that the ejection of black holes is not as efficient
as predicted by most models, 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.
