These Three Supermassive Black Holes Could One Day Join Forces

These Three Supermassive Black Holes Could One Day Join Forces
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Scientists have spotted the best evidence yet of a trio of supermassive black holes in apparently merging galaxies, a new paper reports.

Theories of the universe’s evolution predict that galaxies, and the enormous black holes at their centres, evolve over time by merging with one another. But scenarios where three supermassive black holes exist in the centre of large galaxies are hard to come by. Understanding these systems can help elucidate how galaxies evolve more generally.

“It would really open up our understanding of how galaxy mergers affect supermassive black holes and vice versa,” study author Ryan Pfeifle, Ph.D student at George Mason University, told Gizmodo.

When supermassive black holes begin gobbling up matter and spewing radiation, the become active galactic nuclei. This can happen when two galaxies approach each other. Despite their importance, merging galaxies with dual active galactic nuclei are hard to find, with less than 30 documented candidates, according to the paper in The Astrophysical Journal.

The scientists behind this paper searched for dual active galactic nuclei by first sifting through infrared data on merging galaxies taken by the WISE space telescope. Then, they further studied x-ray data on these sources from the Chandra X-ray Observatory and NuSTAR x-ray space telescope, as well as infrared data from the Large Binocular Telescope and optical data from the Sloan Digital Sky Survey.

One of these sources, called SDSS J0849+1114, seemed to have all the hallmarks of a trio of active galactic nuclei at the centre of merging galaxies. That included three x-ray sources, one at the centre of each of the three galaxies, and a smoking-gun infrared signature of one of the black holes sucking up matter. Combined, this constituted the strongest evidence yet of a trio of supermassive black holes in merging galaxies, Pfeifle said.

SDSS J0849+1114 is exciting for more than just its rarity, since systems like these might help explain why supermassive black holes merge at all. There’s a longstanding astrophysical problem called the “Final Parsec Problem.” When two supermassive black holes get close to one another, they start to orbit and get closer, losing energy via friction with the surrounding stars and gas.

But once they’re approximately a parsec, or 3.26 light-years, apart, simulations show that they take on a more stable orbit, and it would take longer than the age of the universe for them to actually merge. But given that galactic mergers are a part of scientists’ theory of the evolution of the universe, these black holes must merge somehow.

The introduction of a third supermassive black hole to a binary supermassive black hole pair could provide the necessary oomph to allow the merger to happen on a more reasonable timescale.

“They lay out a pretty convincing case that this system is a triple supermassive black hole merger,” Chiara Mingarelli, associate research scientist at the Flatiron Institute Centre for Computational Astrophysics, told Gizmodo. “Seeing this kind of system in real life in the universe is encouraging.”

Scientists are currently hunting for evidence of ripples in spacetime created by supermassive black hole binaries via pulsar timing arrays, experiments that measure the changes to the rate at which dense, rotating neutron stars pulse.

These new results are encouraging, since they provide a way for supermassive black holes to merge and provide hope that pulsar timing array experiments, such as the NANOGrav experiment that’s been running for more than a decade, will find something. However, Mingarelli pointed out that requiring three supermassive black holes to see these waves might delay pulsar timing array results a few years. Hopefully there are some other mechanisms that allow binary supermassive black holes to merge without a third partner, she said.

Pfeifle pointed out that there isn’t a guarantee that these black holes will merge, since that would require a better understanding of the system’s motion.

Still, scientists now have a suite of tools that can find these triple-supermassive black hole systems. Pfeifle said that the team will continue hunting for more examples to develop a more general understanding of them.