Neutrinos are building a ghostly map of the Milky Way

From ghostly particles, astrophysicists have put together a new map of the galaxy in which we live.

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

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

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

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

For as long as humans have looked at the night sky, they have seen the band of diffused light from stars in the galaxy. Advanced telescopes have examined the Milky Way across the entire 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 ejected by a variety of subatomic reactions and are one of the most abundant 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 that one in a million passing neutrinos will hit something, releasing a flash of light that can be picked up by more than 5,000 photomultiplier tubes frozen in 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 the center gobbled up the matter that fell into it.

Somewhat oddly enough, there were no neutrinos that astronomers could say with certainty came from 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 that there were other phenomena that would generate enough high-energy neutrinos to appear in IceCube.

An obstacle to linking neutrinos to events in the Milky Way has been the placement of the IceCube detector in the southern hemisphere, where our galaxy is most easily observed.

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

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

Five years ago she got an idea. Instead of the neutrino signals that astronomers had focused on – long trails of light usefully pointing back to their distant origins – Dr. Kurahashi Neilson will analyze the spherical cascades of light that neutrinos can also generate in IceCube, which are not as useful for determining the origin of the particles.

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

But the blobs aren’t completely symmetrical in all directions — just as a rock thrown into a pond creates ripples that aren’t always exactly circular — so a direction for the neutrino can still be deduced.

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

Steve Sclafani, a graduate student who works 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 of Dortmund in Germany, led the analysis and took advantage of advances in machine learning, a branch of artificial intelligence.

“We are really looking for a needle in a haystack,” said Mr. Hunnefeld.

To avoid fooling themselves, the analysis of 10 years of IceCube data was performed blindly. The researchers did not look at the interim results and the scientists did not know until the end whether their analysis had produced neutrinos from the Milky Way at all. “It was entirely possible that we opened that box and saw zero,” said Dr. Sclafani.

Instead, the analysis turned up hundreds of neutrinos emanating from the galactic plane of the Milky Way. There seems to be some correlation between neutrinos and gamma rays, the highest energy form of light. Both are created in the cascade of particles that come out when high-energy cosmic rays collide with other particles, such as hydrogen gas molecules in interstellar space.

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

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

“We’re seeing all 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 in 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 have the neutrinos as well,” she said, “we can look at those things together to really understand where energetic matter comes from, in our galaxy and beyond.”