A fresh perspective of our galaxy, the Milky Way, is being constructed from the enigmatic particles known as neutrinos. While the current representation remains indistinct and unfinished, the accruing data promises to sharpen the image and shed light on cosmic phenomena like expanding supernova remnants that have remained enigmatic to conventional telescope observations.

“It’s the first occasion we’re visualizing our own galaxy in a medium beyond light,” shares Naoko Kurahashi Neilson, a physics professor from Drexel University, Philadelphia, who conceived the idea of creating a galactic map from neutrinos. Along with her, over 350 researchers collaborated to analyze data from a South Pole neutrino detector, with their findings recently published in the journal Science.

John G. Learned, a physicist from the University of Hawaii not involved in the research, hails this as “the dawn of neutrino astronomy”. From antiquity, humanity has marveled at the scattered light from stars within our galaxy. Advanced telescopes have observed the Milky Way across the light spectrum, from radio waves to ultra-high-energy gamma rays. However, these all remain variants of light.

Neutrinos, on the other hand, are a different story altogether. Born from various subatomic reactions, they are one of the most abundant particles in the cosmos. Yet, their almost negligible weight and sparse interaction with other elements make them unique. At the planet’s southernmost point, scientists have transformed a quarter cubic mile of Antarctic ice into the IceCube Neutrino Observatory. The sheer mass of the ice ensures that one in a million neutrinos passing through will interact, releasing a luminescent flash detected by over 5,000 photomultiplier tubes frozen within the ice.

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Over the past decade, IceCube has identified hundreds of high-energy neutrinos from beyond our solar system. In 2017, other telescopes tracked a neutrino detected by IceCube to a galaxy four billion light-years away. An examination of their data archive revealed that this same galaxy also emitted neutrinos in 2014 and 2015. Last November, IceCube reported detecting roughly 80 neutrinos from NGC 1068, a galaxy merely 47 million light-years from Earth. The suspected source of these neutrinos is the supermassive black hole at the galaxy’s heart, voraciously devouring surrounding matter.

Interestingly, no neutrinos were traced with certainty to our own galaxy, the Milky Way. Although the comparatively tranquil black hole at the center of our galaxy partially explains this, astrophysicists anticipated other events generating enough high-energy neutrinos to be detected by IceCube. A hurdle to connecting neutrinos to Milky Way events was IceCube’s location in the Southern Hemisphere, which is optimal for observing our galaxy.

Dr. Kurahashi Neilson recounts the challenges, saying, “Particles created when high-energy cosmic rays collide with molecules in Earth’s atmosphere overwhelm the usual neutrino signals we seek. It’s almost akin to trying to observe the Milky Way in Los Angeles.” About five years ago, she devised a plan. Instead of focusing on the usual long trails of light pointing to their distant origins, she wanted to investigate the spherical light cascades also produced by neutrinos within IceCube, even though these were less informative about the particles’ origins.

She compares it to a “light blob” and mentions, “In terms of astronomy, we used to disregard it.” However, the blobs, like the uneven ripples from a stone thrown into a pond, could still give a hint to the direction of the neutrinos. Dr. Kurahashi Neilson admits, “Initially, most of my colleagues doubted its feasibility.”

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Steve Sclafani, then a graduate student at Drexel and currently a postdoctoral researcher at the University of Maryland, together with Mirco Hünnefeld, a graduate student at the Technical University Dortmund in Germany, led the analysis. Utilizing advancements in machine learning, a field of artificial intelligence, they searched for the proverbial needle in the haystack.

To ensure objectivity, a blind analysis was conducted on 10 years of IceCube data. The researchers did not preview any intermediate results and were unaware until the end if they had detected any Milky Way neutrinos. Dr. Sclafani remarks, “It was entirely plausible we would find none.”

Contrarily, the analysis revealed hundreds of neutrinos emanating from the galactic plane of the Milky Way. There seems to be some correlation between gamma rays and neutrinos, the most energetic form of light. Both are created in particle cascades, which are collisions between high-energy cosmic rays and other particles, such interstellar hydrogen gas molecules.

There is a hint of a bright spot near the galactic center, potentially neutrinos birthed by the Milky Way’s supermassive black hole, though as Dr. Kurahashi Neilson notes, “it’s not statistically significant”. As more data is accrued, the neutrino emissions from the galaxy’s center will become evident, or they will disappear as a mere statistical aberration.

The cosmic bombardment of gamma rays, neutrinos, and cosmic rays on Earth portrays a universe that’s far from placid, punctuated with exploding stars and black holes engulfing their surroundings.

Regina M. Caputo, an astrophysicist at NASA’s Goddard Space Flight Center, remarks, “We’re observing all these incredibly violent and energetic processes”. Elizabeth A. Hays, the project scientist for NASA’s Fermi Gamma-Ray Space Telescope, predicts that IceCube will offer a new, distinctive view. “Now that we also have the neutrinos,” she says, “we can jointly examine these to truly understand the origins of energetic matter in our galaxy and beyond.

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