Our Milky Way galaxy is an awe-inspiring feature of the night sky, dominating all wavelengths of light and viewable with the naked eye as a horizon-to-horizon hazy band of stars. Now, for the first time, the IceCube Neutrino Observatory has produced an image of the Milky Way using neutrinos—tiny, ghostlike astronomical messengers.
In an article to be published tomorrow, June 30, in the journal Science, the IceCube Collaboration, an international group of over 350 scientists, presents evidence of high-energy neutrino emission from the Milky Way.
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. The Wisconsin IceCube Particle Astrophysics Center (WIPAC), a research center at UW–Madison, is the lead institution for the IceCube project.
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.
“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 and 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.”
Gamma-rays have long been detected from the galactic plane, but it has remained unclear whether they come from energetic electrons scattering light of the Milky Way or protons and heavier nuclei interacting with galactic gas. The detection of high-energy neutrinos from the galactic plane provides a clear answer to this question, because high-energy neutrinos can only come from protons.
The IceCube search focused on the southern sky, where the bulk of neutrino emission from the galactic plane is expected near the center of our galaxy. However, until now, the background of muons and neutrinos produced by cosmic-ray interactions with the Earth’s atmosphere posed significant challenges made more so by the relatively sparse neutrino production.
To overcome them, IceCube collaborators at Drexel University 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, developed by IceCube collaborators at TU Dortmund University, 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 dataset used in the study included 60,000 neutrinos spanning 10 years of IceCube data, 20 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 observation of the galactic plane with IceCube has profound implications. Halzen and UW–Madison colleagues Ke Fang and Jay Gallagher’s subsequent analysis of the IceCube result indicates that the Milky Way is ten to a hundred times dimmer in neutrinos than the average of distant galaxies. This may be an important clue to solve the ongoing mystery of precisely where and how extremely high-energy cosmic rays are produced.
“One implication is that our galaxy has not hosted the type of sources that produced the bulk of high-energy neutrinos for the past few million years,” says Fang, “which is roughly the time since the last jet activity of the black hole of our own galaxy. Planned and future follow-up analyses by IceCube will further our understanding of the particle accelerators of our own galaxy.”
To learn more about these results, join UW–Madison physics professor Justin Vandenbroucke on July 5th at 7:00 PM for Wednesday Nite @ The Lab.
The IceCube Neutrino Observatory is funded and operated primarily through an award from the National Science Foundation to the University of Wisconsin–Madison. 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. https://icecube.wisc.edu/collaboration/institutions. IceCube’s 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 NSF. IceCube construction was also funded with significant contributions from the National Fund for Scientific Research (FNRS & FWO) in Belgium; the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG) in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the Wisconsin Alumni Research Foundation in the U.S.
+ info “Observation of high-energy neutrinos from the Galactic plane,” The IceCube Collaboration: R. Abbasi et al. DOI:10.1126/science.adc9818
Francis Halzen, IceCube Principal Investigator
Vilas Research Professor and Gregory Breit Distinguished Professor of Physics
Wisconsin IceCube Particle Astrophysics Center, University of Wisconsin–Madison
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