Scientists have shed new light on a major astronomical mystery by tracing elusive “ghost” particles from outer space back to a dusty galaxy located 47 million light years from Earth, reports a new study.
By linking the ghost particles—called neutrinos—to this galaxy, researchers are one step closer to solving a century-old question about the origin of energetic particles, known as cosmic rays, that travel across space at close to light speed. Cosmic rays, which produce “astrophysical” neutrinos with very high energies, are crucial clues to unraveling the secrets of the most explosive and luminous phenomena in the universe.
Scientists used the IceCube Neutrino Observatory, a special telescope that extends for more than a mile under the Antarctic ice at the South Pole, to capture roughly 80 astrophysical neutrinos from a galaxy known as NGC 1068, or Messier 77, which has an extremely active galactic core. The finding suggests that these active galaxies provide “a substantial contribution” to the abundance of astrophysical neutrinos, and therefore cosmic rays, that permeate through the universe, according to a study published on Thursday in Science.
“This is a very exciting result because for the first time, we actually understand that astrophysical neutrinos can be related to this very special type of galaxy,” said Theo Glauch, an experimental physicist at the Technical University of Munich and a co-author of the new study, in a call with Motherboard. “We physicists call them active galaxies because they're very different from, for example, our Milky Way.”
Unlike our own galaxy, which is currently dormant, NGC 1068 contains “an extremely bright environment which we can only study in neutrinos,” Glauch added. “Neutrinos are the only particles that can directly escape from the processes that drive this extremely high luminosity in the core of those galaxies.”
In this way, the new results open a new window into NGC 1068, which contains a type of extremely energetic core called an active galactic nucleus (AGN) that is powered by a supermassive black hole. Though NGC 1068 has been known to skywatchers for centuries, many of its secrets have remained hidden from view due to a cloud of dust that surrounds its center.
The intense environment around AGN can warp the paths of cosmic rays, and even forms of light such as energetic gamma rays. Neutrinos, in contrast, are so lightweight that they can simply glide out of galaxies like NGC 1068 without getting tossed around by turbulent galactic forces, earning them the nickname ghost particles. Because they pretty much just travel in a straight line, astrophysical neutrinos can point to the sources of cosmic rays, though their slippery nature also makes them extremely difficult to detect on Earth.
Enter: IceCube, the largest neutrino telescope on Earth. The detector is made up of thousands of sensors that reach deep into the Antarctic ice, providing an ultra-still environment that can capture astrophysical neutrinos. It took nearly a decade to snag the 80 neutrinos that the collaboration has linked to NGC 1068, which were collected from 2011 to 2020. Many other astrophysical neutrinos were also sensed by Ice Cube during this period, but their sources remain unknown at this time.
The achievement marks the second time that scientists have identified a point source of high-energy neutrinos using IceCube. In 2018, the IceCube Collaboration, which is composed of more than 400 scientists around the world, announced that it had tracked a single neutrino back to a type of radiant galactic core called a “blazar” located four billion light years from Earth. (A study published in 2020 also singled out NGC 1068 as a significant source of neutrinos, but the new results have confirmed that finding with far greater accuracy.)
“Last time, we only had a single very high-energy neutrino,” said Glauch, referring to the 2018 study. “This time we observed an accumulation of 80 neutrino events. We studied the emissions at different energies, which really has a lot of more information.”
“The fundamental idea is that through the neutrinos, we learn how those AGNs are working,” he added. “In this case, the machine that's driving the emission is completely different from the one that we had in the previous findings.”
These astrophysical neutrinos, and their cosmic ray progenitors, are imbued with staggering amounts of energy—far exceeding what is possible in the best particle accelerators on Earth—but it’s unclear where these particles come from or how they got so juiced up. Now, IceCube has shown that blazers and AGN are responsible for at least some of these high-energy particles.
People observed those cosmic rays more than 100 years ago, and we’ve never understood where they come from,” Glauch said. He noted that finding out the origin of cosmic rays and neutrinos, together, can shed light on the high-energy universe, which contains the highest energy radiation ever seen. “It’s pretty much curiosity to understand how those things are related and how they are related also to the evolution of the universe and the different objects which we have which we receive, like active galaxies, for example,” he continued.
To that end, the collaboration hopes to identify other point sources of neutrinos by re-examining its existing dataset, while also working to capture many more of these particles in the future. By continuing to study these ghost particles from space, the researchers could build an entirely new map of the high-energy universe, and the exotic objects that fuel it.
“At IceCube, we know that we have many more astrophysical neutrinos somewhere hidden in our data,” Glauch said. “We have probably resolved around 1 percent of the astrophysical neutrinos that we see, so there's still 99 percent, at least, to discover. They must come from some other objects—most likely from similar objects—or there are other surprises. So, looking into the future, the challenge is definitely to improve on the data, improve on the analysis, but then also to have new and bigger detectors that can collect more data, to resolve the rest of what we call the the isotropic astrophysical neutrino flux, which is still very much unknown [in terms of] where it comes from.”
“We have some hints now, which is very nice, because they also tell us where to look in the future,” he concluded. “But there's still a long way to go to really understand this entire mystery.”