Three supermassive black holes have been spotted merging into one

Three galaxies with supermassive black holes at their centers appear to be merging into a single giant galaxy, a process that astronomers have rarely observed.

To reach such enormous sizes, astronomers believe that supermassive black holes must sometimes devour or merge with other massive black holes in collisions between galaxies. This process is difficult to spot, both because these mergers are short-lived compared to the lifespan of the black hole and because black holes can only be easily seen if they emit light while actively feeding on matter, which is also rare. As a result, astronomers have only captured about 150 pairs of merging galactic black holes.

Now Emma Schwartzman of the US Naval Research Laboratory in Washington DC and colleagues have discovered 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 becomes,” says Schwartzman.

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

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

There are already visible signs that the galaxies have begun to interact with each other, says Isabella Lamperti of 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 about 70,000 light-years and the third is 300,000 light-years away.

But compared to their total lifespan of billions of years, we are witnessing the end of history. “It’s like we’re reliving the final moments of a soap opera about a galaxy merger,” says Emma Kun of Ruhr University in Bochum, Germany.

She said it is extremely difficult to simulate the merger of three active supermassive black holes, but observing this system will allow physicists to better understand what happens in more complex mergers. “This is the first step to discovering physics about the system,” says Kun.

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