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Astronomers discover twin black holes

Dual discovery A pair of black holes have been found inside a globular cluster, a surprise finding that has astronomers rethinking what goes on inside these stellar cities.

The black holes were detected in M-22, a 12 billion year old globular cluster located more than 10,000 light-years from Earth.

While it is thought that hundreds of black holes can form in the typical globular cluster, astronomers believe all but one would be ejected through a series of gravitational interactions.

Study co-author Dr James Miller-Jones of the International Centre for Radio Astronomy Research at Curtin University in Western Australia says they were initially searching for the presence of a black hole in a nearby cluster.

"They've been detected in other galaxies, so we theorised there might be a black hole in the centre of this cluster," says Miller-Jones, "but we never expected to find two."

Miller-Jones and colleagues were using the Very Large Array radio telescope in New Mexico to look for a central black hole in the cluster, which would be identified by its radio waves emissions.

The locations were then compared to images of the same regions taken by NASA's Earth-orbiting Chandra x-ray telescope.

"Both black holes are actively feeding off companion stars", says Miller-Jones.

"Matter falling into the black hole emits x-rays which are detected by Chandra. The ratio of radio waves to x-rays gave us their mass which tells us they're likely to be black holes."

Extra big

Reporting in the journal Nature, Miller-Jones and colleagues say both black holes have masses of between ten and twenty times that of the Sun.

"This makes each about two to three times more massive than other stellar mass black holes," says Miller-Jones.

"We think that means they formed in a slightly different way because of the cluster environment they're in."

"It also explains why their radio wave emissions were so bright," says Miller-Jones.

The researchers believe the discovery could mean M-22 contains a hundred yet to be detected black holes.

"There could be other black holes in the cluster, but they're not actively feeding, so we don't see them," says Miller-Jones.