Neutrinos reveal hidden secrets of Milky Way

The Milky Way galaxy, a breathtaking sight in the night sky, can now be observed in a groundbreaking manner. The IceCube Neutrino Observatory has generated an image of the Milky Way using neutrinos, peculiar and elusive astronomical particles. An article set to be published in the esteemed journal Science unveils the IceCube Collaboration’s findings, which provide evidence of the emission of high-energy neutrinos from our very own galaxy.

These high-energy neutrinos possess energies that surpass those generated by stellar fusion reactions by millions to billions of times. The IceCube Neutrino Observatory, a colossal detector stationed at the Amundsen-Scott South Pole Station, successfully detected these ghostly particles. Its design encompasses a cubic kilometer of deep Antarctic ice, meticulously equipped with over 5,000 light sensors. By scouring the cosmos, IceCube seeks to identify indications of high-energy neutrinos originating from within our galaxy and extending to the farthest reaches of the universe.

“What makes this discovery particularly fascinating is that, unlike light of any wavelength, the universe appears more radiant in neutrinos, outshining even the nearby sources within our own galaxy,” explains Francis Halzen, the principal investigator of IceCube and a physics professor at the University of Wisconsin–Madison.

A multi-messenger view of the Milky Way galaxy, centered on the galactic center and viewed in galactic coordinates. Each panel shows the entire Galactic plane in a band of ±15◦ in galactic latitude, with each panel having a unique color scale. The panels, from top to bottom, are: 1) view in the optical range, which is partly obscured by clouds of gas and dust that absorb optical photons, 2) the integrated flux in gamma rays as seen by the Fermi-LAT 12 year survey, 3) emission template for the expected neutrino flux, taken to match the template from Fermi-LAT measurements, 4) emission template from panel 3 convolved with the IceCube detector acceptance for cascade-like neutrino events and 5) pre-trial significance of the all-sky scan for point-like sources using the cascade neutrino event sample in the same band of the Galactic plane. Credit: IceCube

Denise Caldwell, the director of NSF’s Physics Division, highlights the indispensable role of technological advancements in driving significant scientific breakthroughs. She emphasizes that the remarkable capabilities of the highly sensitive IceCube detector, in conjunction with cutting-edge data analysis tools, have unveiled an entirely fresh perspective of our galaxy—a perspective that had merely been hinted at in the past. As these capabilities continue to evolve and refine, we can eagerly anticipate witnessing a progressively sharper and more detailed rendition of our galaxy, potentially uncovering concealed characteristics that have eluded human observation until now.

Interactions occurring between cosmic rays—energetic protons and heavier atomic nuclei, which are also generated within our galaxy—and the interstellar gas and dust inevitably produce two distinctive outcomes: gamma rays and neutrinos. Considering the previous detection of gamma rays emanating from the galactic plane, it was reasonably anticipated that the Milky Way would also serve as a source of high-energy neutrinos.

The neutrino view (blue sky map) in front of an artist’s impression of the Milky Way. Credit: IceCube Collaboration/Science Communication Lab for CRC 1491

According to Steve Sclafani, a physics Ph.D. student at Drexel University, IceCube member, and co-lead analyzer, the confirmation of a neutrino counterpart measurement validates our existing knowledge about the Milky Way and its cosmic ray sources.

The investigation primarily concentrated on the southern hemisphere of the sky, as it is anticipated to host the majority of neutrino emissions from the galactic plane near the center of our galaxy. However, grappling with the considerable challenges posed by the background of muons and neutrinos generated by interactions between cosmic rays and the Earth’s atmosphere was essential.

To surmount these obstacles, the IceCube collaborators at Drexel University devised sophisticated analyses that specifically targeted “cascade” events—neutrino interactions in the ice that produce nearly spherical patterns of light. By focusing on cascade events, where the energy deposition initiates within the instrumented volume, the contamination from atmospheric muons and neutrinos was effectively minimized. Consequently, the enhanced purity of the cascade events substantially improved the sensitivity to astrophysical neutrinos originating from the southern skies.

Two images of the Milky Way galaxy. The top is captured with visible light and the bottom is the first-ever captured with neutrinos. Credit: IceCube Collaboration/U.S. National Science Foundation (Lily Le & Shawn Johnson)/ESO (S. Brunier)

However, the pivotal breakthrough emerged through the integration of machine learning techniques developed by IceCube collaborators at TU Dortmund University. These methods significantly enhance the identification of neutrino-induced cascades, as well as improve the accuracy of direction and energy reconstruction. The detection of neutrinos originating from the Milky Way stands as a testament to the growing significance of machine learning in data analysis and event reconstruction within the IceCube project.

“The implementation of these advanced methods enabled us to preserve more than ten times the number of neutrino events while achieving superior angular reconstruction. As a result, our analysis exhibited three times greater sensitivity compared to previous searches,” explains Mirco Hünnefeld, an IceCube member, physics Ph.D. student at TU Dortmund, and co-lead analyzer.

The study harnessed a comprehensive dataset comprising 60,000 neutrino events spanning a decade of IceCube observations. This represents thirty times the quantity of events considered in a prior analysis focused on the galactic plane using cascade events. The identified neutrinos were then compared to previously published prediction maps, which delineate regions in the sky where neutrino emissions from the Milky Way were anticipated.

A view of the IceCube Lab with a starry night sky showing the Milky Way and green auroras. Credit: Yuya Makino, IceCube/NSF

The maps employed in the study encompassed various sources of information. One map was generated by extrapolating observations from the Fermi Large Area Telescope, which focuses on gamma-ray emissions in the Milky Way. Additionally, two alternative maps, known as KRA-gamma, were provided by a group of theorists who developed them.

Wolfgang Rhode, an IceCube member, professor of physics at TU Dortmund University, and advisor to Mirco Hünnefeld, emphasizes the significance of employing modern machine learning methods in knowledge discovery, citing the long-awaited detection of cosmic ray interactions within our galaxy as a prime example of their efficacy.

The potential of machine learning extends far into the future, enabling scientists to approach and explore other observations that were previously out of reach.

The confirmation of the Milky Way as a source of high-energy neutrinos, supported by compelling evidence that has withstood thorough scrutiny by the IceCube collaboration, marks a significant milestone. The subsequent step involves identifying specific sources within our galaxy, an endeavor that will be addressed in forthcoming analyses conducted by IceCube.

Naoko Kurahashi Neilson, an IceCube member, professor of physics at Drexel University, and advisor to Steve Sclafani, highlights the monumental nature of observing our own galaxy through the lens of particles rather than light. As the field of neutrino astronomy continues to evolve, it promises to provide a new perspective through which we can observe and understand the universe.

Source: University of Wisconsin-Madison

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