Unexpected pair of black holes confounds experts

By AAP with AG staff 5 October 2012
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A globular cluster of stars with two black holes settled at its core has sent astronomers back to the drawing board.

ASTRONOMERS HAVE FOUND evidence of something they never expected to see – two black holes lurking inside a 12-billion-year-old cluster of tightly-packed stars in our Milky Way galaxy.

The international team – including researchers at Curtin University in Western Australia – was taken by surprise when they found what appeared to be two black holes, each about 10-20 times more massive than our Sun, near the core of a star cluster named Messier 22 (M22).

A black hole is what is left when a massive star dies and collapses in on itself – they are very dense regions in space-time, with a gravitational force so strong that even light cannot escape. Scientists who study them hope to learn more about the evolution of galaxies.

Challenging accepted theory of cluster evolution

The new finding, reported today in the journal Nature, challenges much of the accepted science on globular star clusters, which are about 10 billion years old on average and each contain around a million stars. Theorists had believed that only one matter-sucking black hole could exist in a globular cluster, of which there are dozens in the Milky Way.

“We were searching for one large black hole in the middle of the cluster, but instead found two smaller black holes a little way out from the centre,” says study co-author James Miller-Jones at Curtin University’s International Centre for Radio Astronomy Research in Perth. “[This] means all the theory and simulations need refinement.”

Simulations of star cluster evolution had shown that many black holes are created early in a cluster’s history. Scientists had believed the black holes sink towards the middle of the cluster where they start a gravitational ‘dance’ – most of them being flung out of the cluster with only one remaining.

Actively feeding black holes

But the team said their evaluation of X-ray, radio, optical and infrared images of M22 led them to conclude that the two objects they spotted were indeed black holes.

Commenting on the findings in a commentary also published in Nature, Dr Stefan Umbreit an astrophysicist at the Northwestern University in Illinois, USA, said the team’s interpretation of the data was “compelling”.

If correct, the pair of black holes in M22 is the first to be found in a globular cluster in our galaxy. M22 is about 10,000 light-years from Earth. “M22 may contain as many as 100 black holes but we can’t detect them unless they’re actively feeding on nearby stars,” Miller-Jones said in a statement.

Black holes normally lurk dormant and undetected at the centre of galaxies, but can occasionally be tracked by the scraps left over when they feast on stars that venture too close.