Scientists have revealed a uniquely different picture of the Milky Way by determining the galactic origin of thousands of neutrinos, invisible “ghost particles” that exist in large quantities but normally pass through the Earth undetected, as published in the specialized journal Science. This image, the first of its kind, constitutes a galactic portrait made with particles of matter instead of electromagnetic energy.
From visible light from stars to radio waves, the Milky Way has long been observed through the different frequencies of electromagnetic radiation it emits. Now the advance has been possible thanks to the collaboration of researchers using the IceCube Neutrino Observatory at the NSF Amundsen-Scott Station in Antarctica, within the territory of the South Pole.
The massive observatory detects the subtle signals of high-energy neutrinos from space using thousands of networked sensors buried deep within a cubic kilometer of clear, pristine ice.
“At this point in human history, we are the first to see our galaxy in any way other than light,” explains Naoko Kurahashi Neilson, a physicist at Drexel University, referring to the moment when she and two students from Ph.D., Steve Sclafani of Drexel, United States, and Mirco Hünnefeld, of the Technical University of Dortmund, Germany, first examined the image.
From technology to science
Kurahashi Neilson proposed the innovative computational analysis used to generate the image and received funding to carry out his idea through a grant from the NSF Faculty Early Career Development program.
“As is often the case, technological advances make great scientific advances possible,” said Denise Caldwell, director of the NSF Physics Division. The possibilities offered by the highly sensitive IceCube detector, together with new data analysis tools, “have given us a completely new view of our galaxy, which until now has only been hinted at,” she adds.
“As these capabilities continue to be perfected,” this scientist continues her explanation, “we can expect to see this image emerge with ever-increasing resolution, potentially revealing hidden features of our galaxy never before seen by humanity.” »The intriguing thing is that, unlike what happens with light of any wavelength, in neutrinos the universe eclipses the nearby sources of our own galaxy«, says Francis Halzen, a physicist at the University of Wisconsin-Madison and IceCube principal investigator.
Find the origin
Beyond the challenge of detecting the elusive neutrinos and distinguishing them from other types of interstellar particles, there is the even more ambitious goal of determining their origin. When neutrinos interact with the ice beneath IceCube, those rare encounters produce faint light patterns that IceCube can detect.
Some light patterns are highly directional and clearly point to a specific area of the sky, allowing researchers to determine the source of the neutrinos. These interactions were the basis for the discovery in 2022 of neutrinos from another galaxy 47 million light years away.
Other interactions are much less directional and produce “cascading balls of light” in the transparent ice, explains Kurahashi Neilson. His IceCube colleagues, Sclafani and Hünnefeld, developed a machine learning algorithm that compared the relative position, size and energy of more than 60,000 neutrino-generated light cascades recorded by IceCube over 10 years.
The three researchers spent more than two years meticulously testing and verifying their algorithm with artificial data simulating neutrino detections. When they finally fed the real data provided by IceCube into the algorithm, the result was an image showing bright spots corresponding to places in the Milky Way suspected of emitting neutrinos.
These were places where the observed gamma rays were thought to be byproducts of collisions between cosmic rays and interstellar gas, which should theoretically also produce neutrinos. “Now a homologous neutrino has been measured, thus confirming what we know about our galaxy and the sources of cosmic rays,” emphasizes Sclafani.
Over many decades, scientists have revealed countless astronomical discoveries by expanding the methods used to observe the universe. Once revolutionary advances such as radio astronomy and infrared astronomy have been joined by a new class of observation techniques that use phenomena such as gravitational waves and, now, neutrinos.
Kurahashi Neilson says the neutrino-based image of the Milky Way is another step in that lineage of discoveries. He predicts that neutrino astronomy will be refined like the methods that preceded it, until it too can reveal hitherto unknown aspects of the universe.