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Astronomers Find Evidence of “Missing Link” Black Hole

An international team of astronomers has announced new evidence for the existence of a middleweight black hole in space – the “missing link” between stellar mass and supermassive black holes.

The intermediate-mass black hole, announced in Nature, has 2200 times the mass of our Sun and hides at the centre of the 12-billion-year-old globular 47 Tucanae star cluster, 13,000 light-years from Earth in the southern constellation of Tucana the Toucan.

The 47 Tucanae cluster contains three millions stars in a sphere about 120 light-years in diameter. It also holds about two dozen pulsars that were important targets of the investigation.

The new research relied on two lines of evidence. The first is overall motions of stars throughout the cluster.

A/Prof Holger Baumgardt of The University of Queensland, who conducted computer simulations used in the research project, said that a globular cluster’s environment was so dense that heavier stars tended to sink to the centre of the cluster. “An intermediate-mass black hole at the cluster’s centre acts like a cosmic ‘spoon’ that stirs the pot, causing the stars near it to slingshot to higher speeds and greater distances, imparting a subtle signal that astronomers can measure,” he said.

By employing computer simulations of stellar motions and distances, and comparing them with visible-light observations, the team found evidence for just this sort of gravitational stirring.

Baumgardt said the second line of evidence came from pulsars, compact remnants of dead stars whose radio signals were easily detectable. These objects were also flung about due to the gravitational influence of the central intermediate-mass black hole, causing them to be found at greater distances from the cluster’s centre than would be expected if no black hole existed.

Combined, this evidence suggested the presence of an intermediate-mass black hole of about 2200 solar masses within 47 Tucanae. The fact this black hole had eluded detection for so long suggests that black holes of similar masses could be hiding in other globular clusters.