Three supermassive black holes have been spotted merging into one

Three galaxies with supermassive black holes at their centres appear to be in the process of merging into a single, giant galaxy, a process that astronomers have rarely seen.

To grow to such enormous sizes, astronomers think supermassive black holes must occasionally devour or merge with other massive black holes during collisions between galaxies. This process is difficult to spot, both because these mergers are short-lived compared with the lifetime of the black hole and because the black holes can only be easily seen if they are giving off light from actively feeding on material, which is also rare. As a result, astronomers have only caught around 150 pairs of galactic black holes in the act of merging.

Now, Emma Schwartzman at the US Naval Research Laboratory in Washington DC and her colleagues have found a group of three supermassive black holes, all actively feeding, that appear to be combining into a single system. “The more galaxies involved, the rarer the system gets,” says Schwartzman.

Each of the supermassive black holes is emitting low-frequency radiation in the form of radio waves, which can pass through dust that blocks other light. This allowed Schwartzman and her team to observe them with two radio observatories, the Very Long Baseline Array in Hawaii and the Very Large Array in New Mexico, then rule out that the light was coming from another source, like bright galaxies full of stars.

“What’s really interesting is that all three of these [black holes] emit in the radio regime, which we’ve never seen before,” says Schwartzman. “There’s no guarantee that any [black hole] would emit in the radio regime.”

There are already visible signs that the galaxies have begun interacting with each other, says Isabella Lamperti at the University of Florence, Italy, but they are at a relatively early stage of interaction, given that two of the galaxies are still separated by around 70,000 light years and the third is 300,000 light years away.

But relative to their total lifetime of billions of years, we are witnessing the end of the story. “It’s like catching the final moments of a galaxy-merging soap opera,” says Emma Kun at Ruhr University Bochum in Germany.

Simulating how three active supermassive black holes merge is extremely difficult, she says, but observing this system will allow physicists to better understand what happens in more complex mergers. “This is the first step of finding some physics about the system,” says Kun.

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