Two stellar-mass black holes in the globular cluster M22

TL;DR

This paper reports the discovery of two stellar-mass black holes in the globular cluster M22, challenging existing models that predict most black holes are ejected from such clusters, and suggests a larger population of black holes may exist there.

Contribution

The study provides the first observational evidence of multiple stellar-mass black holes within a single globular cluster, indicating black hole ejection is less efficient than previously thought.

Findings

Detected two black holes via radio sources in M22

Black holes are more massive than typical stellar remnants

M22 may host 5-100 black holes in total

Abstract

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.