The IceCube Neutrino Observatory has produced an image of the Milky Way galaxy using neutrinos. The groundbreaking image is the first of its kind and provides critical evidence that the Milky Way is a source of high-energy neutrinos.
What is a Neutrino?
Before grappling with the significance of the incredible work performed at the IceCube Neutrino Observatory, it is important to discuss the nature of a neutrino briefly.
“A neutrino is a subatomic particle that is very similar to an electron but has no electrical charge and a very small mass, which might even be zero,” explains Scientific American. “Neutrinos are one of the most abundant particles in the universe. Because they have very little interaction with matter, however, they are incredibly difficult to detect.”
To detect neutrinos, scientists use enormous and sensitive detectors. A low-energy neutrino will typically travel through many light-years of matter before interacting with anything. Hence, neutrino detection experiments on Earth often measure very few neutrinos in a massive detector. For example, Scientific American notes that the Sudbury Neutrino Observatory, which has a 1,000-ton heavy water solar-neutrino detector, detects about 30 neutrinos per day.
IceCube Neutrino Observatory
“The high-energy neutrinos, with energies millions to billions of times higher than those produced by the fusion reactions that power stars, were detected by the IceCube Neutrino Observatory, a gigaton detector operating at the Amundsen-Scott South Pole Station. It was built and is operated with National Science Foundation (NSF) funding and additional support from the fourteen countries that host institutional members of the IceCube Collaboration. This one-of-a-kind detector encompasses a cubic kilometer of deep Antarctic ice instrumented with over 5,000 light sensors. IceCube searches for signs of high-energy neutrinos originating from our galaxy and beyond, out to the farthest reaches of the universe,” explains the IceCube Neutrino Observatory in a press release.
In November 2013, IceCube detected 28 neutrinos that likely originated outside the Solar System. Some of these neutrinos were extremely high-energy, including the most energetic neutrinos ever observed at the time, subsequently named “Bert” and “Ernie.” Later in the year, “Big Bird” broke the record.
The IceCube Collaboration, comprised of more than 350 scientists, announced in 2018 that it had traced a high-energy neutrino back to its origination point in the blazar TXS 0506 +056, a quasar located 5.7 billion light-years from Earth. This marked the first time a neutrino detector had been able to identify an object in space and indicated a source of cosmic rays.
In November 2022, the IceCube Collaboration detected a neutrino source emitted by the active galactic nucleus of Messier 77, a barred spiral galaxy located about 47 million light-years from Earth.
The New View of the Milky Way Through the Lens of Neutrinos
After more than a decade of monumental scientific observations, the IceCube Neutrino Observatory’s latest discovery might be its most impressive. Using data collected by the observatory, the IceCube Collaboration has delivered its incredible view of the Milky Way galaxy “painted” with neutrino particles, marking the first time that the galaxy has been observed using a particle rather than different wavelengths of light.
The Milky Way, and many other cosmological structures, have been observed in various wavelengths of light, including infrared bands, visible light, X-rays, gamma rays, radio waves, and more.
Neutrinos are generated within the Milky Way when cosmic rays, which are high-energy particles or atom fragments that originate outside the solar system and travel through the interstellar medium at nearly the speed of light, collide with interstellar matter. Neutrinos are also generated by stars, and probably by exploding stars, although that is not a settled matter. Neutrinos provide scientists with a unique view of extremely high-energy events in the Milky Way galaxy and the rest of the universe that cannot be seen solely using light.
The IceCube Neutrino Observatory’s sensors are buried several kilometers below the extremely clear Antarctic ice. The observatory relies upon this ice under immense pressure to detect faint radiation emitted by charged particles. “The particles are created by incoming neutrinos, which come from cosmic ray collisions in the galaxy, hitting the atoms in the ice,” writes The Conversation.
Neutrinos, often called ghostly particles, have been striking the IceCube Neutrino Observatory’s sensors since construction was completed. However, not all neutrinos have an astrophysical origin — some neutrinos result from cosmic rays interacting and colliding with the Earth’s atmosphere. While undoubtedly interesting, these local-origin neutrinos need to be separated from neutrinos from deep space.
The researchers in the IceCube Collaboration narrowed their focus to a specific type of neutrino interaction called a cascade. These interactions are roughly spherical “showers of light,” and provide more sensitivity to astrophysical neutrinos from the Milky Way. A cascade interaction is harder to reconstruct, but it gives a more accurate measurement of a neutrino’s energy, which is a strong indicator of its origin.
“Analysis of ten years of IceCube data using sophisticated machine learning techniques yielded nearly 60,000 neutrino events with an energy above 500 gigaelectronvolts (GeV). Of these, only about 7% were of astrophysical origin, with the rest being due to the ‘background’ source of neutrinos that are generated in the Earth’s atmosphere,” writes The Conversation.
The team’s results reject the hypothesis that all measured neutrino events result from interactions in Earth’s atmosphere. However, the research has not quite met the standard 5 sigma standard for statistical significance in particle physics. That said, the team says that its result has only a 1 in 150,000 chance of being a fluke.
“What’s intriguing is that, unlike the case for light of any wavelength, in neutrinos, the universe outshines the nearby sources in our own galaxy,” says Francis Halzen, a professor of physics at the University of Wisconsin-Madison, the principal investigator of IceCube.
“As is so often the case, significant breakthroughs in science are enabled by advances in technology,” says Denise Caldwell, director of NSF’s Physics Division. “The capabilities provided by the highly sensitive IceCube detector, coupled with new data analysis tools, have given us an entirely new view of our galaxy — one that had only been hinted at before. As these capabilities continue to be refined, we can look forward to watching this picture emerge with ever-increasing resolution, potentially revealing hidden features of our galaxy never before seen by humanity.”
The team used machine learning methods developed by IceCube collaborators at TU Dortmund University to improve the detection and identification of cascades produced by neutrinos. The analytical methods also delivered accurate directional data and allowed for energy reconstruction.
“The improved methods allowed us to retain over an order of magnitude more neutrino events with better angular reconstruction, resulting in an analysis that is three times more sensitive than the previous search,” says IceCube member, TU Dortmund physics Ph.D. student, and co-lead analyzer Mirco Hünnefeld.
The Next Steps
“The strong evidence for the Milky Way as a source of high-energy neutrinos has survived rigorous tests by the collaboration,” says Ignacio Taboada, a professor of physics at the Georgia Institute of Technology and IceCube spokesperson. “Now the next step is to identify specific sources within the galaxy.”
The IceCube Collaboration plans additional observations and analyses, and a planned enlargement, IceCube Gen2, will be 10 times bigger than the current experiment. The team says this will allow it to turn “the current blurry picture” into a detailed view of the Milky Way galaxy.
“Observing our own galaxy for the first time using particles instead of light is a huge step,” explains Naoko Kurahashi Neilson, professor of physics at Drexel University and IceCube member. “As neutrino astronomy evolves, we will get a new lens with which to observe the universe.”