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Pair of supermassive black holes discovered

23/04/14Science & Technology

Global: A pair of supermassive black holes orbiting around one another have been discovered by an international research team from the Max Planck Institute for Radio Astronomy in Bonn, Germany.

This is the first time such a pair have been found in an ordinary galaxy. They were discovered because they ripped a star apart when the European Space Agency’s (ESA’s) space observatory XMM-Newton was looking in their direction.

Most massive galaxies in the Universe are thought to harbour at least one supermassive black hole at their centre. Two supermassive black holes are the proof that the galaxy has merged with another. Finding binary supermassive black holes can tell astronomers about how galaxies evolved into their present day shapes and sizes.

To date, only a few candidates for close binary supermassive black holes have been found. All are in active galaxies where they are constantly ripping gas clouds apart, before crushing them out of existence.

“There might be a whole population of quiescent galaxies that host binary black holes in their centres,” said Dr Stefanie Komossa, from the Max Planck Institute. But finding them is a difficult task because in quiescent galaxies, there are no gas clouds feeding the black holes, and so the cores of these galaxies are truly dark.

The only hope that astronomers have is to be looking in the right direction at the moment one of the black holes rips a star to pieces. Such an occurrence is called a ‘tidal disruption event’. As the star is pulled apart by the gravity of the black hole, it gives out a flare of X-rays.

On 10 June 2010, a tidal disruption event was spotted by XMM-Newton in galaxy SDSS J120136.02+300305.5, approximately 2 billion light years away. Komossa and her colleagues were scanning the data for such events and scheduled follow-up observations just days later with XMM-Newton and NASA’s Swift satellite.

The galaxy was still spilling X-rays into space. It looked exactly like a tidal disruption event caused by a supermassive black hole but the X-rays fell below detectable levels between days 27 and 48 after the discovery. Then they re-appeared and continued to follow a more expected fading rate, as if nothing had happened.

Now this behaviour can be explained. “This is exactly what you would expect from a pair of supermassive black holes orbiting one another,” says Professor Fukun Liu from Peking University in China.

The findings are published in Astrophysical Journal.

Pan European Networks Ltd