We may finally know the source of mysterious high-energy neutrinos

Supermassive black holes at the hearts of active galaxies may be churning out a lot of the universe’s high-energy neutrinos.

Two teams using data from IceCube, the world’s premier neutrino observatory located in Antarctica, have independently identified a common type of these active galaxies, called Seyfert galaxies, as likely neutrino producers. These findings, reported in Physical Review Letters and arXiv.org, bolster some astronomers’ view that the cores of such active galaxies could churn out the majority of the cosmic neutrinos seen streaming across the universe.

“I would say that they can be the major contributor,” says astronomer Andrii Neronov of the Astroparticle and Cosmology Laboratory in Paris, who is a coauthor of the study in Physical Review Letters. “I would put a reasonable bet on them now with the information that I know.”

This wasn’t the case just a few years ago. When astronomers first identified Seyfert galaxy NGC 1068 as the likely origin of high-energy neutrinos in 2022, it came as a surprise to some. Many astronomers at the time didn’t think this type of galaxy could produce neutrinos. Now, with compelling evidence that neutrinos are coming not only from NGC 1068 but from two other galaxies like it, NGC 4151 and NGC 3079, there’s little doubt that active galaxies — defined as those with supermassive black hole cores belching immense amounts of energy, called active galactic nuclei, can be capable neutrino producers.

“It is exciting to see active galactic nuclei emerging as a class of neutrino sources, especially the ones obscured by a surrounding layer of dust and gas as a source of high-energy neutrinos,” says astrophysicist Sreetama Goswami of the University of Nevada at Las Vegas, and a coauthor on the new results in the June 10 arXiv.org preprint submitted to Astrophysical Journal Letters.

The new findings add to a flurry of recently published theoretical research that has made headway in tracking down the exact origins of cosmic neutrinos. Several studies, including a paper accepted to Nature Astronomy and published as a preprint at arXiv.org on May 30, have pinpointed the region around the central supermassive black hole in NGC 1068 as the only place in the galaxy where the neutrinos could be produced. If NGC 4151 and NGC 3079 are confirmed as neutrino factories, their high-energy particles would likely also be produced near their central black holes.

Previously, scientists have only confidently identified one other significant extragalactic neutrino emitter: a blazar (SN:07/12/18). These cosmic giants are also active galaxies, but they have enormous jets of supercharged material where the neutrinos are thought to be produced. Now, however, some astronomers think that maybe it’s the blazar’s black hole that is ultimately responsible.

“I think that dense cores, not jets, produce neutrinos,” says Francis Halzen, principal investigator of IceCube and a coauthor on the June 10 arXiv.org paper. “These can be in Seyferts, flat-spectrum radio quasars [types of blazars], or anything else with an obscured black hole.”

Kohta Murase, a theoretical physicist at Penn State University, who has done extensive research to identify the source of NGC 1068’s neutrinos, has long thought the black hole hearts of these galaxies are probable neutrino mills. “One of the promising sites for neutrino production is the corona, which is a very hot region surrounding the black hole,” he says. “If this is established, it might give us a clue to understanding the physical properties of the corona.”

Along with discussing where neutrinos are produced in active galaxies, astronomers are also debating what types of active galaxies are the most important neutrino producers. Some scientists assert jetless active galactic nuclei galaxies, like Seyferts, could produce nearly all extragalactic neutrinos. Other researchers maintain blazars are necessary to explain particularly high-energy neutrinos. Or perhaps there are more unidentified neutrino-spewing objects yet to be discovered.

The consensus among astronomers is that active galaxies with supermassive black holes are probably a big source of high-energy neutrinos, but exactly how big is still unknown.