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Almost all galaxies are known to have a single supermassive black hole at their heart.

But scientists have now found that one galaxy actually has three, and it could be a precursor to more such galaxies being discovered.

The discovery could also help astronomers in the search for gravitational waves - the 'ripples in spacetime' predicted by Einstein.

Astronomers have discovered three closely orbiting supermassive black holes in a galaxy more than 4 billion light years away. Illustrated here are helix-shaped jets from one supermassive black hole caused by its very closely orbiting companion, while the third black hole emits relatively straight jets due to its distance

The finding, published in Nature, was made by an international team led by Dr Roger Deane from the University of Cape Town.

They examined six systems thought to contain two supermassive black holes.

The team found that one of these called SDSS J1502+1115 over 4 billion light years away contained three supermassive black holes.

This is the tightest trio of black holes detected at such a large distance, with two of them orbiting each other at just 500 light years apart rather like binary stars.

The finding suggests that these closely-packed supermassive black holes - each with a mass between 1 million to 10 billion times that of the sun - are far more common than previously thought.

And the three supermassive black holes could also provide further observations of elusive gravitational waves.

This is the tightest trio of black holes known to date and is remarkable since most galaxies have just one, usually with a mass between 1 million to 10 billion times that of the Sun, at their centre. The discovery suggests that such closely packed supermassive black holes (illustration shown) are far more common than thought

This particular discovery was made using a technique called Very Long Baseline Interferometry (VLBI) to discover the inner two black holes of the triple system.

This technique combines the signals from large radio antennas around the world separated by up to 6,200 miles (10,000 kilometres) to see detail 50 times finer than that possible with the Hubble Space Telescope.

This discovery was made with the European VLBI Network, an array of European, Chinese, Russian and South African antennas, as well as the 305 metre Arecibo Observatory in Puerto Rico.

Future radio telescopes, like the Square Kilometre Array (SKA), will be able to further measure the gravitational waves from such black hole systems.

Gravitational waves from cosmic inflation generate a faint but distinctive twisting pattern in the universe. Shown here is the pattern observed with the Bicep2 telescope, providing evidence for cosmic inflation after the Big Bang, although the results have been called into question

'What remains extraordinary to me is that these black holes, which are at the very extreme of Einstein's Theory of General Relativity, are orbiting one another at 300 times the speed of sound on Earth,' said Deane.

'Not only that, but using the combined signals from radio telescopes on four continents we are able to observe this exotic system one third of the way across the universe.

'It gives me great excitement as this is just scratching the surface of a long list of discoveries that will be made possible with the Square Kilometre Array (SKA).'

Professor Matt Jarvis of Oxford University's Department of Physics, an author of the paper, added: 'General Relativity predicts that merging black holes are sources of gravitational waves and in this work we have managed to spot three black holes packed about as tightly together as they could be before spiralling into each other and merging.

'The idea that we might be able to find more of these potential sources of gravitational waves is very encouraging as knowing where such signals should originate will help us try to detect these "ripples" in spacetime as they warp the universe.'