Milky Way a Source of 'Ghost" Particles

The Milky Way inspires awe in the night sky, a hazy, glowing band of stars. Adding to its mystique, scientists have now discovered that tiny, energetic “ghost” particles, called neutrinos, emanate from our home galaxy.

The finding was made by the international IceCube Collaboration, which includes more than 350 scientists, including a team from the University of Delaware. The research is published in the June 30 issue of the journal Science.

“We now know that our closest source of high-energy neutrinos is right in our own cosmic backyard,” said David Seckel, professor of physics and astronomy. Seckel is leader of the UD team involved in the multi-university collaboration, which is headquartered at the University of Wisconsin.

Neutrinos are of great interest to scientists because they provide a new lens for studying the universe using particles as a probe, Seckel said. These high-energy particles carry information from distant, cataclysmic events like exploding stars, and the direction from which these particles arrive points back to their source.​

Since neutrinos are electrically neutral, they don’t interact much with other matter and can zip through space, planets and even our own bodies unimpeded and undetected. This property makes them notoriously difficult to study, requiring the development of specialized detectors — with the IceCube Neutrino Observatory at the South Pole the very largest in the world.

Neutrinos with energies millions to billions of times higher than those produced by the fusion reactions that power stars were detected coming from the Milky Way by this novel gigaton detector embedded in the Antarctic ice.

The one-of-a-kind detector encompasses a cubic kilometer of Antarctic ice instrumented with over 5,000 light sensors. When a neutrino crashes into an atomic nucleus in the ice, a burst of blue light, called Cherenkov light, is emitted. This shimmering light is captured by the detector’s sensors, and a neutrino is “caught.”

Tracing neutrinos to their source

Confirming the Milky Way as a source of neutrinos is the latest page in a 100-year story, Seckel said, noting that neutrinos were predicted in 1930 to explain features of radioactivity, discovered in the lab in 1956 and explored at high-energy accelerators from 1962.

High-energy neutrinos produced by cosmic ray interactions in Earth's atmosphere have been studied since the 1980s and are used to refine our understanding of the fundamental nature of matter. IceCube was conceived to detect high-energy astrophysical neutrinos produced in concert with the highest energy cosmic rays.

This is the third instance where the IceCube collaboration has successfully traced high-energy neutrino emissions detected by the IceCube Observatory to a point source. The previous two were made outside our galaxy.

The first, confirmed in 2017 and shared with the world in 2018, connected neutrino emissions to blazar TXS 0506+056, a giant galaxy powered by a black hole over 5 billion light years away. The second confirmation, announced in November 2022, directly linked neutrino emissions to NGC 1068, also known as Messier 77, an active galaxy in the constellation Cetus, 47 million light years away from Earth.

Spencer Axani, assistant professor of physics and astronomy at UD, is part of the team working on improvements in data processing and calibration for IceCube.

“We always expected a signal from our galaxy and now we’ve found it,” Axani said of this latest advance.

He anticipates many more discoveries, with the increased use of new data science tools.

“IceCube is a great collaboration — there’s a lot of hands working on these improvements, and our discoveries are really going to accelerate with machine learning,” Axani said.​

Data analyses by Drexel University and the implementation of machine learning tools by TU Dortmund University were critical to identifying high-energy neutrinos, their direction and energy reconstruction for the Milky Way. This work allowed the team to retain over an order of magnitude more neutrino events, resulting in an analysis that was three times more sensitive than previous searches.

The dataset used in the study included 60,000 neutrinos spanning 10 years of IceCube data — 30 times as many events as the team had used in previous analyses.

“Over the past four to five years, it’s clear that the data analysis techniques are just better,” Seckel said. “We’re improving what we can do and what we can learn. This applies not just to IceCube, but to the use of machine learning and artificial intelligence in general. These tools are incredibly important and relevant. The people getting their graduate degrees from IceCube institutions are going off into the broader marketplace.”

Digging deeper into the data

But exactly where are those neutrinos coming from in the Milky Way?

“It’s difficult to be able to narrow it to one source or another yet,” Seckel said. “There’s a large black hole at the center of the galaxy, but this central engine is not tremendously active at the moment.”

With the application of new data science techniques, the IceCube Collaboration is likely to have the answer soon.

After all, who wouldn’t want to know how and where a “ghost” particle got blasted into space?

For Axani, he’s intrigued by the conundrum of neutrinos being the most abundant particles in the universe, yet humanity’s ability to detect so few of them so far.

“We have to develop these behemoth detectors to even see them,” Axani said. “This research allows us to probe the fundamental properties of our Universe. So far, neutrinos are the only particle to point to physics beyond the Standard Model, the basic building blocks of the universe. Digging deeper requires very interesting science and cool technology, and we’re all about that.”

The IceCube Collaboration, with over 350 scientists in 58 institutions from around the world, runs an extensive scientific program that has established the foundations of neutrino astronomy. Its research efforts, including critical contributions to the detector operation, are funded by agencies in Australia, Belgium, Canada, Denmark, Germany, Italy, Japan, New Zealand, Republic of Korea, Sweden, Switzerland, Taiwan, the United Kingdom, and the United States, including the National Science Foundation.​

Article by Tracey Bryant, with resources from IceCube Collaboration 

Photos courtesy of Spencer Axani

June 29, 2023