Neutrinos Build a Ghostly Map of the Milky Way

From ghostlike particles, astrophysicists have pieced together a new map of the galaxy we live in.

For now, that map of the Milky Way is blurry and incomplete. But as more data is gathered, it will become clearer and will help illuminate galactic convulsions like the expanding remnants of exploded stars, providing clues to mysteries that are difficult to solve with only observations from conventional telescopes.

“This is the first time we’ve seen our own galaxy in anything other than light,” said Naoko Kurahashi Neilson, a professor of physics at Drexel University in Philadelphia who came up with the idea that a new view of the galaxy could be gleaned from particles known as neutrinos.

Dr. Kurahashi Neilson and the more than 350 other scientists who collaborate on analyzing data from a neutrino detector at the South Pole reported their findings in a paper published on Thursday in the journal Science.

“This is at last really the beginning of neutrino astronomy,” said John G. Learned, a physicist at the University of Hawaii who was not involved with the research.

For as long as people have looked up at the night sky, they have seen the band of diffuse light from stars within the galaxy. Sophisticated telescopes have surveyed the Milky Way across the spectrum of light, from radio waves to ultra-high-energy gamma rays. But those are all forms of light.

Neutrinos are completely different: They are particles that are ejected from a variety of subatomic reactions and are one of the most common particles in the universe. But they weigh next to nothing and rarely interact with anything.

At the bottom of the earth, scientists have turned a quarter cubic mile of Antarctic ice into the IceCube Neutrino Observatory. The ice provides enough mass so that one out of every million neutrinos passing through will hit something, releasing a flash of light that can be captured by more than 5,000 photomultiplier tubes frozen into the ice.

Last November, the IceCube team reported the detection of about 80 neutrinos from NGC 1068, a galaxy just 47 million light-years from Earth. Those neutrinos were most likely spewed out as the supermassive black hole at its center voraciously gobbled up matter falling into it.

Somewhat curiously, there were no neutrinos that astronomers could confidently say had come from within our Milky Way galaxy. In some ways, that was not surprising. The black hole at the center of the Milky Way is much quieter than that of NGC 1068. But astrophysicists expected there were other phenomena that would generate enough high-energy neutrinos to show up in IceCube.

One obstacle to linking neutrinos to events in the Milky Way was the placement of the IceCube detector in the Southern Hemisphere, where our galaxy is most readily observed.

“You would think it’s better because the detector is in the Southern Hemisphere,” Dr. Kurahashi Neilson said. But instead, particles created when high-energy cosmic rays hit molecules in Earth’s atmosphere wash out the neutrino signal astronomers usually look for.

“It’s almost like trying to see the Milky Way in Los Angeles,” Dr. Kurahashi Neilson said.

Around five years ago, she had an idea. Instead of the neutrino signals the astronomers had focused on until then — long tracks of light that helpfully point back to their distant origin — Dr. Kurahashi Neilson wanted to analyze the spherical cascades of light that neutrinos can also generate within IceCube, which are not as helpful for determining the origins of the particles.

“It’s like a blob of light,” she said. “We used to just throw it away in terms of astronomy.”

But the blobs are not completely symmetric in all directions — just as a rock thrown into a pond creates ripples that are not always exactly circular — so a direction for the neutrino could still be inferred.

“I think most of my collaborators didn’t believe this was viable back then,” Dr. Kurahashi Neilson said. “You want to push the envelope, but you don’t want to do something that’s impossible to do. So you have all these ideas that are at the edge, and you have to pick one that you think might actually work.”

Steve Sclafani, a graduate student working with Dr. Kurahashi Neilson at Drexel who is now a postdoctoral researcher at the University of Maryland, and Mirco Hünnefeld, a graduate student at the Technical University Dortmund in Germany, spearheaded the analysis, taking advantage of advances in machine learning, a branch of artificial intelligence.

“We’re really doing a needle-in-the-haystack search,” Mr. Hünnefeld said.

To avoid the possibility of deceiving themselves, the analysis of 10 years of IceCube data was performed blind. The researchers did not look at any of the intermediate results, and the scientists did not know until the end whether their analysis had turned up any Milky Way neutrinos at all. “It was fully possible that we opened up that box and we saw zero,” Dr. Sclafani said.

Instead, the analysis turned up hundreds of neutrinos that came from the galactic plane of the Milky Way. There appears to be some correlation between neutrinos and gamma rays, the highest energy form of light. Both are created in the cascade of particles that spill out when high-energy cosmic rays slam into other particles like hydrogen gas molecules in interstellar space.

There is a suggestive bright spot near the galactic center — perhaps neutrinos generated by the Milky Way’s supermassive black hole — but “it’s not as statistically significant,” Dr. Kurahashi Neilson said. As more data is collected, neutrino emissions from the center of the galaxy will become distinct — or it will fade because it was just a statistical fluke.

The showering of cosmic rays, gamma rays and neutrinos on Earth shows the universe is anything but calm, with exploding stars, and black holes swallowing their surroundings.

“We’re seeing all of these incredibly violent and energetic processes,” said Regina M. Caputo, an astrophysicist at NASA’s Goddard Space Flight Center in Maryland who was not involved with the IceCube project.

Elizabeth A. Hays, the project scientist for NASA’s Fermi Gamma-Ray Space Telescope, said IceCube will provide a new and different view. “Now that we also have the neutrinos,” she said, “we can look at those things together to really understand where is energetic matter coming from, in our galaxy and beyond it.”

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