Scientists surprised to find three black holes merging into each other in strange, nearby galaxy

Scientists have spotted a full three black holes swirling in a strange nearby galaxy.

Astronomers had long thought that the smashed up set of stars known as NGC 6240 formed when two galaxies crashed into each other, with the collision creating two black holes. But in fact there are three, scientists have found, each of which are merging at the same time.

The discovery helps shed light on the formation of the universe's biggest galaxies and changes our understanding of the phenomenon. It is an unprecedented view, marking the first time that three supermassive black holes have been spotted so close to each other anywhere in the universe.

Massive galaxies like our own Milky Way are usually host to not only hundreds of billions of stars, but also a black hole that can have a mass equivalent to many hundreds of millions of our Sun. Our own galaxy has its own supermassive black hole, which remains mysterious to scientists.

The galaxy known as NGC 6240 – which is relatively near to us, just 300 million light years away – is not like that. It is known as an "irregular galaxy" because of its strange shape, which makes it look spread out and smashed.

Until now, researchers have thought that shape was the consequence of its strange birth, which they thought was the result of two smaller galaxies colliding with each other and leaving two black holes in the middle. Those black holes are sliding into each other at a pace of several hundreds of kilometres per second.

Researchers thought they knew a lot about the relatively nearby galaxy because they have been able to look at it in such detail, and were so well known they had become a very important object of study for our understanding of the formation and structure of galaxies. So they were surprised to find the two black holes in the middle were actually three.

"Through our observations with extremely high spatial resolution we were able to show that the interacting galaxy system NGC 6240 hosts not two - as previously assumed - but three supermassive black holes in its centre," said Wolfram Kollatschny from the University of Göttingen, the lead author of the study.

What's more, the three are closer together than ever seen before. They are squashed together in a region that measures less than 3,000 lightyears across, which represents less than one per cent of the total size of the galaxy.

"Up until now, such a concentration of three supermassive black holes had never been discovered in the universe," said Dr Peter Weilbacher of the Leibniz Institute for Astrophysics Potsdam (AIP). "The present case provides evidence of a simultaneous merging process of three galaxies along with their central black holes."

The discovery could help change our understanding of how galaxies form. Until now, we have not been able to explain how the biggest galaxies formed over the roughly 14 billion years the universe has been around.

"If, however, simultaneous merging processes of several galaxies took place, then the largest galaxies with their central supermassive black holes were able to evolve much faster," said Peter Weilbacher. "Our observations provide the first indication of this scenario."