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A pair of nearby supermassive black holes are heading for a collision

Two supermassive black holes have been discovered just 1600 light years apart, and they are likely to collide in about 250 million years

Space 30 November 2021

Close-up view of the two bright galactic nuclei, each housing a supermassive black hole, in NGC 7727, a galaxy located 89 million light-years away from Earth in the constellation Aquarius. Each nucleus consists of a dense group of stars with a supermassive black hole at its centre. The two black holes are on a collision course and form the closest pair of supermassive black holes found to date. It is also the pair with the smallest separation between two supermassive black holes ??? observed just 1600 light-years apart in the sky.???? The image was taken with the MUSE instrument on ESO???s Very Large Telescope (VLT) at the Paranal Observatory in Chile.

The two bright spots in this image are dense groups of stars, each with a supermassive black hole at the centre

ESO/Voggel et al.

A pair of relatively nearby supermassive black holes are getting ready to collide. These enormous objects are closer together than any supermassive pair we have spotted before, as well as being the closest pair to Earth.

Karina Voggel at the University of Strasbourg in France and her colleagues found these two behemoths using the Very Large Telescope in Chile. The larger black hole has a mass 154 million times that of the sun, and the smaller one weighs in at 6.3 million solar masses.

They are in a galaxy called NGC 7727, which is about 89 million light years from Earth. The larger black hole sits at the centre of the galaxy, while the smaller one is about 1600 light years to the side – it probably once belonged to a smaller galaxy that was swallowed up billions of years ago by NGC 7727.


Now, the black holes themselves are heading towards a merger. The researchers calculate that they are likely to smash together and merge into a single colossal black hole in about 250 million years.

“These processes in astronomy take billions of years, so we can’t follow them as they happen, but we’ve caught this in the act of the merging process,” says Voggel. “This is a phase that we aren’t typically able to observe very often.”

The two objects are more than five times closer to Earth than the next-closest supermassive black hole pair, which is why the researchers were able to find them. They spotted the black holes based on the fast movements of nearby stars, not on the light that some black holes give off as matter falls in, which is the usual method.

“In this case, the black holes are silent – they’re not very loud – and that’s how these have been overlooked,” says Voggel. “There may be many more supermassive black holes hidden outside of the centres of galaxies.” She estimates that accounting for off-centre black holes like the secondary one in this system could increase the number of known supermassive black holes in the universe by as much as 30 per cent.

Journal reference: Astronomy & Astrophysics, DOI: 10.1051/0004-6361/202140827

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