Observations of high-energy astrophysical neutrinos have shown that they mostly originate from extragalactic sources such as active galaxies. However, gamma ray observations show bright emission from within our Milky Way Galaxy, and astrophysical gamma rays and neutrinos are expected to be produced by the same physical processes. The IceCube Neutrino Observatory — a cubic-kilometer particle detector built deep within Antarctic ice — searched for neutrino emission from within the Milky Way and found evidence of extra neutrinos emitted along the Galactic plane, which is consistent with the distribution of gamma-ray emission. The results imply that high-energy neutrinos can be generated by nearby sources within the Milky Way.
An artist’s composition of the Milky Way seen through a neutrino lens (blue). Image credit: IceCube / NSF / Lily Le / Shawn Johnson / ESO / S. Brunier.
“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,” said University of Wisconsin-Madison’s Professor Francis Halzen, principal investigator of the IceCube Collaboration.
Interactions between cosmic rays — high-energy protons and heavier nuclei, also produced in our Galaxy — and galactic gas and dust inevitably produce both gamma rays and neutrinos.
Given the observation of gamma rays from the Galactic plane, the Milky Way was expected to be a source of high-energy neutrinos.
“A neutrino counterpart has now been measured, thus confirming what we know about our Galaxy and cosmic ray sources,” said IceCube member Steve Sclafani, a Ph.D. student at Drexel University.
The search focused on the southern sky, where the bulk of neutrino emission from the Galactic plane is expected near the Milky Way’s center.
However, until now, the background of muons and neutrinos produced by cosmic-ray interactions with the Earth’s atmosphere posed significant challenges.
To overcome them, the IceCube team developed analyses that select for cascade events, or neutrino interactions in the ice that result in roughly spherical showers of light.
Because the deposited energy from cascade events starts within the instrumented volume, contamination of atmospheric muons and neutrinos is reduced.
Ultimately, the higher purity of the cascade events gave a better sensitivity to astrophysical neutrinos from the southern sky.
However, the final breakthrough came from the implementation of machine learning methods that improve the identification of cascades produced by neutrinos as well as their direction and energy reconstruction.
The observation of neutrinos from the Milky Way is a hallmark of the emerging critical value that machine learning provides in data analysis and event reconstruction in IceCube.
“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,” said IceCube member Mirco Hünnefeld, a Ph.D. student at TU Dortmund University.
The dataset included 60,000 neutrinos spanning 10 years of IceCube data, 30 times as many events as the selection used in a previous analysis of the Galactic plane using cascade events.
These neutrinos were compared to previously published prediction maps of locations in the sky where the Galaxy was expected to shine in neutrinos.
The maps included one made from extrapolating Fermi Large Area Telescope gamma-ray observations of the Milky Way and two alternative maps identified as KRA-gamma by the group of theorists who produced them.
“This long-awaited detection of cosmic ray-interactions in the Galaxy is also a wonderful example of what can be achieved when modern methods of knowledge discovery in machine learning are consistently applied,” said TU Dortmund University’s Professor Wolfgang Rhode, member of the IceCube Collaboration.
The power of machine learning offers great future potential, bringing other observations closer within reach.
“The strong evidence for the Milky Way as a source of high-energy neutrinos has survived rigorous tests by the collaboration,” said Georgia Institute of Technology’s Professor Ignacio Taboada, spokesperson of the IceCube Collaboration.
“Now the next step is to identify specific sources within the Galaxy.”
These and other questions will be addressed in planned follow-up analyses by IceCube.
“Observing our own Galaxy for the first time using particles instead of light is a huge step,” said Drexel University’s Professor Naoko Kurahashi Neilson, member of the IceCube Collaboration.
“As neutrino astronomy evolves, we will get a new lens with which to observe the Universe.”
The findings were published in the journal Science.
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IceCube Collaboration. 2023. Observation of high-energy neutrinos from the Galactic plane. Science 380 (6652): 1338-1343; doi: 10.1126/science.adc9818
