The globular cluster M22 is a dense swarm of hundreds of thousands of stars, hanging in our galaxy like a glowing beehive. In such a crowded environment, stars often collide and swap partners, but massive objects like black holes were thought to be quickly ejected by gravitational interactions. New research shows that one black hole has managed to stay.
Astronomers pointed three instruments at M22: the VLA radio telescope, the Hubble Space Telescope, and the Chandra X-ray Observatory. Among eight detected radio sources, VLA22 stood out: its radio and X-ray brightness fell exactly on the line that typically identifies quiescent black holes paired with a star. Its radio spectrum is moderately steep, and its X-ray emission becomes harder — consistent with systems where a black hole slowly accretes material from a companion star.
Near VLA22, two possible companion stars were spotted: a bright main-sequence star and a fainter subgiant or red giant star. This looks like a binary system with a fairly long orbital period — the star and black hole circle each other slowly, like two dancers holding hands on a spacious stage.
The discovery of VLA22 supports recent models suggesting that stellar-mass black holes can remain in globular clusters for the entire age of the Universe. Additionally, another source, VLA19, showed an inverted radio spectrum — typical of a compact jet emanating from the vicinity of a black hole. A third source, VLA40, also fell on the expected line, but further observations are needed to be certain.
The globular cluster M22 is visible even in a small telescope: it lies in the constellation Sagittarius and appears as a fuzzy patch, though it actually consists of hundreds of thousands of stars.