A unique observatory buried in pristine Antarctic ice detected a stream of elusive neutrino particles streaming from the center of a distant galaxy obscured with dust.
The observation by the IceCube observatory on the South Pole is only the second detection of a source of cosmic neutrinos, and scientists hope it may shed light on what's going on inside supermassive black holes.
Neutrinos are odd. They are everywhere but most of the time, they don't interact with other particles or any type of matter. That's because they have very little mass and no electric charge. For this reason, they are incredibly difficult to spot. But their total indifference to their surroundings also means that unlike other particles, they don't get distracted from their path, cruising across vast distances in straight lines from their sources. That means that once astronomers know how to detect them, they can track neutrinos to their origins much more easily than other types of particles.
An international team of scientists has now reported detecting such a stream of neutrinos from a galaxy known as NGC 1086 (which is sometimes called Messier 77 or the Squid Galaxy). NGC 1086 is a dusty galaxy, with a shape quite similar to that of the Milky Way. NGC 1086, however, produces bursts of stars at a much higher rate than our galactic home and swirls around a black hole much more massive than the one at the Milky Way's center.
This hungry black hole, devouring huge amounts of material, forms a core of a sparkling active galactic nucleus that gives off bright bursts of high-energy cosmic rays and charged particles that outshine the galaxy's stars. Most of the black hole's crackle is, however, obscured from view because the galaxy's center is hidden by a thick ring when viewed from Earth. The neutrinos, however, with their ability to pass through matter, escape this ring and reach our planet undisturbed.
"We are peering inside active regions of the NGC 1068 galaxy 47 million light-years away," Gary Hill, an associate professor of physics at the University of Adelaide in Australia and one of the authors of the paper, said in a statement. "As we observe neutrinos emitted by it we will be able to learn more about the extreme particle acceleration and production processes occurring inside the galaxy, which hasn't been possible up to now as other high energy emissions can't escape from it."
The detection makes NGC 1068 only the second source of cosmic neutrinos ever identified. In 2018, the IceCube observatory found a stream of neutrinos coming from an active galactic nucleus of a galaxy known as TXS 0506+056.
That galaxy, located in the constellation Orion, is 100 times farther away from Earth than NGC 1068 but emits a jet of material at nearly the speed of light, which points directly at Earth. That makes any radiation coming from TXS 0506+056 much easier to spot than that from NGC 1068.
"After the excitement in 2018 of the discovery of neutrinos from TXS 0506+056, it's even more thrilling to find a source producing a steady stream of neutrinos that we can see with IceCube," Hill said. "The fact that neutrinos can escape from within these otherwise-obscured regions of the universe means they are also hard to detect."
The IceCube observatory is a unique installation. It consists of over 5,000 detectors submerged at depths of 0.9 to 1.5 miles (1.5 to 2.5 kilometers) in the pristine Antarctic ice. Suspended on 86 vertical cables that are spaced 410 feet apart (125 meters), the detectors register tiny flashes of blue light triggered when highly energetic neutrinos crash into the atomic nuclei of the molecules of ice.
The observatory, built in the 2000s, has been operational since 2010. The recent study analyzed detections of high-energy neutrinos made between 2011 to 2020, looking for possible sources of those particles among known active galaxies. Computer modeling previously suggested that active black holes, such as that at the center of NGC 1068, must be able to accelerate particles and eject them into the intergalactic space together with bursts of high-energy radiation. Scientists expect other similar galaxies to produce their very own neutrino streams.
"One neutrino can single out a source. But only an observation with multiple neutrinos will reveal the obscured core of the most energetic cosmic objects," Francis Halzen, a professor of physics at the University of Wisconsin–Madison and principal investigator of the IceCube project, said in a separate statement. "IceCube has accumulated some 80 neutrinos of teraelectronvolt energy from NGC 1068, which are not yet enough to answer all our questions, but they definitely are the next big step towards the realization of neutrino astronomy."
Astronomers are currently planning a second generation IceCube detector that will be able to detect a thousand times more neutrinos and spot five times fainter sources. Gradually, the astronomers said, the obscured universe will open up, leading to a new era in astronomy.
NGC 1068 could become a "standard candle" for this future neutrino research, Theo Glauch, a postdoctoral associate at the Technical University of Munich (TUM) in Germany and co-author of the paper, said in the statement. The galaxy, discovered in 1780, is well known to astronomers and has been studied for centuries.
The study is published in the journal Science on Nov. 4.