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Scientists pin down the spin of a snacking black hole

Paul Sutherland, News Editor
30 July 2013, 12:07

Sen— Black holes are, as their name suggests, invisible, exotic objects that are a gateway out of our own Universe and from which light cannot escape.

However, they make their presence known by the effects they have on other nearby objects, such as discs of debris that swirl around them and the high-energy jets of radiation they emit.

Supermassive black holes, which are found at the hearts of most galaxies, are of great interest to astronomers because they hold clues to how those vast cosmic cities of stars have evolved.

Now a team based at Durham University in the UK has made a breakthrough in supermassive black hole research by finding a new way to measure how fast they spin. It could lead to a better understanding of how they drive galaxy growth.

The team, led by Professor Chris Done, zoomed in on a black hole that is 10 million times the mass of the Sun and which lies at the centre of a spiral galaxy 500 million light years from us in the constellation of Virgo.

They observed a surrounding disc of gas and dust on which the black hole was feeding to fuel its growth and to power its activity. Their observations, using the European Space Agency’s XMM-Newton satellite, looked at what was happening to this so-called accretion disc in the optical, ultra-violet and soft x-ray regions of the spectrum, and allowed the team to measure how far the disc was from the black hole.

This distance depends on the speed at which a black hole spins because one that spins rapidly draws the disc in close to itself, the astronomers say.

Professor Chris Done led the research at Durham’s Department of Physics. She told Sen: “It is tricky to talk about the spin of a black hole because it doesn’t have a surface. But you can talk about the speed at which spacetime itself is rotating at the black hole’s event horizon - the boundary where even light cannot escape.

“The spin of this black hole comes out to be low, nowhere close to the maximum spin rate it could possibly have. It is going round at less than half the speed of light, which is at least a factor of two slower than a maximally spinning black hole, where the horizon goes around at the speed of light.

“This tells us that the black hole grows by ‘snacking’, where it builds up its mass (and hence its spin) by catching multiple small bits of gas which come in from different directions rather than gobbling a huge lump that all comes in from one direction. In a sense, the black hole spin is kind of a fossil record of how the black hole grows its mass.”

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Artist's impression of the XMM-Newton spacecraft in orbit around the Earth. Credit: ESA/D. Ducros

Supermassive black holes can spit out incredibly hot particles at high energies that prevent intergalactic gases from cooling and forming new stars in the outer galaxy. Though scientists don’t yet understand why the jets are ejected into space, the Durham experts believe that their power could be linked to the spin of the black hole.

Professor Done said: “We know the black hole in the centre of each galaxy is linked to the galaxy as a whole, which is strange because black holes are tiny in relation to the size of a galaxy. This would be like something the size of a large boulder influencing something the size of the Earth.

“Understanding this connection between stars in a galaxy and the growth of a black hole, and vice-versa, is the key to understanding how galaxies form throughout cosmic time.

“If a black hole is spinning it drags space and time with it and that drags the accretion disc, containing the black hole’s food, closer towards it. This makes the black hole spin faster, a bit like an ice skater doing a pirouette.

“By being able to measure the distance between the black hole and the accretion disc, we believe we can more effectively measure the spin of black holes.

“Because of this, we hope to be able to understand more about the link between black holes and their galaxies.”