Two galaxies with supermassive black holes are set to engage in a "monster galactic mashup".

The galaxies, called Arp 299, were spotted by Nasa's Nuclear Spectroscopic Telescope Array (NuStar) telescope 134 million light years away.

NuStar is the first telescope able of finding where high-energy X-rays are coming from within the tangled pair. Findings show that one of the supermassive black holes is actively "gorging on gas" while the other is "snoozing away".

Andrew Ptak of Nasa's Goddard Space Flight Centre and author of the study that will be published in the Astrophysical Journal, said: "When galaxies collide, gas is sloshed around and driven into their respective nuclei, fuelling the growth of black holes and the formation of stars.

"We want to understand the mechanisms that trigger the black holes to turn on and start consuming the gas."

arp 299
In the centre panel, the NuStar high-energy X-ray data appear in various colours overlaid on a visible-light image from Nasa's Hubble Space Telescope. The panel on the left shows the NuStar data alone, while the visible-light image is on the far right NASA/JPL-Caltech/GSFC

Every large galaxy has a supermassive black hole at its core. These black holes have masses of more than one million suns put together.

Previous studies of Arp 299 had indicated the presence of the two supermassive black holes, but without NuStar's capabilities, if one or both of them were feeding – where the black hole bulks up in mass as it drags gas onto it.

However, they have now been able to confirm that the supermassive black hole on the right is very hungry, feeding on gas that results in electrons and protons being heated to hundreds of millions of degrees. The result is a superhot plasma that boosts the visible light to high-energy X-rays.

The other black hole is not so greedy, however. It is seen to be in a dormant state - or, it is buried in so much gas and dust that its high-energy X-rays cannot escape.

Ann Hornschemeier, a co-author of the study, said: "Odds are low that both black holes are on at the same time in a merging pair of galaxies. When the cores of the galaxies get closer, however, tidal forces slosh the gas and stars around vigorously, and, at that point, both black holes may turn on."

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