This artist’s concept shows the Milky Way as observed with neutrinos (blue) rather than light. Credit: IceCube Collaboration/US National Science Foundation (Lily Le & Shawn Johnson)/ESO (S. Brunier)
In the vast expanses of space, neutrinos move silently, rarely interacting with the myriad particles that foul their path. Some of these many billions of ghostly particles end in a faint flash of light, illuminating an ice cube the size of an arena at the South Pole.
Equipped with well-placed detectors and aptly named the IceCube Neutrino Observatory, this array now allows astronomers to image the Milky Way – not with light, but with particles. The resulting image was first published on June 29 Science.
Seeing in neutrinos
Naoko Kurahashi Neilson, a particle physicist at Drexel University in Philadelphia, has worked at IceCube for more than 10 years. Operating in Antarctica, IceCube is a collaboration of hundreds of scientists from 50 institutions and 14 countries. And unlike the common telescopes you may be familiar with, which use light (photons) to see the universe, IceCube uses a different type of particle: the neutrino.
Neilson and her team have now reconstructed the Milky Way using IceCube data from the past decade. “This is the first time we’ve seen our galaxy in anything other than light,” she says.
Both photons and neutrinos are fundamental particles. “When you see the night sky with your own eyes, photons from every star hit all the way through space, after traveling millions of kilometers to hit your eyeballs, and that’s where they end up. That’s how you know there’s a star there,” says Neilson. Most telescopes work the same way, only they can see objects that are fainter than the human eye.
And with IceCube, Neilson says, “we do the same thing with neutrinos.”
Observing with neutrinos offers enormous advantages. The dust that penetrates galaxies and envelops black holes scatters so many photons that standard telescopes cannot see beyond it. Due to their non-interactive nature, neutrinos allow us to penetrate through the dust and observe galaxies or celestial bodies that we would otherwise not be able to see. “When the telescope creates an image of the universe, the neutrino observatory creates X-rays,” says Christina Love, also an IceCube collaborator at Drexel University.
Astronomy on ice
However, neutrinos are difficult to detect because they rarely interact with matter. IceCube addresses this problem by literally using a giant ice cube. “And what better place to do that than Antarctica?” Neilson says. The telescope collects most of its data during the polar winter, when planes don’t land or take off. About 40 scientists and staff remain on the totally isolated base for five months a year. “You don’t see another soul or even the sun!” Neilson says.
“When you’re in Antarctica, you stand at 2 miles [3 kilometers] made of ice,” she explains. “We drilled detectors into the bottom third of this ice, where it’s pitch black.” Due to the dense nature of the ice, high-energy neutrinos from space hit the atomic nuclei in the ice and break up into a series of high-energy particles that emit light. The detectors then easily capture this light in an otherwise completely dark area. A machine learning algorithm uses information to reconstruct where the neutrino came from in space, such as which detectors light up when and how intense the light is. Researchers use it to try to map the universe.
A new perspective
Since IceCube’s inception in 2011, the observatory has recorded over a million neutrinos. Surprisingly, none of the neutrinos previously identified by astronomers came from the Milky Way. This is due to the way the observations were made. When a neutrino passes the detector, it leaves a linear, trace-like light path that is used to determine the neutrino’s origin. But neutrinos that leave this signature belong to distant objects, outside our galaxy.
Neilson came up with the idea of looking for a different kind of neutrino signal, a cascading “blob of light”. These signals make it difficult to determine exactly where the neutrino came from, and thus have been generally ignored in previous identifications of cosmic neutrinos. Analyzing these blob-like neutrino paths was like looking for a needle in a haystack, and the team wasn’t sure they would find meaningful results.
However, the new search turned up hundreds of neutrinos that appear to have come from the plane of the Milky Way. Converted into an image, it shows the regions of our galaxy that produce high-energy neutrinos.
In addition, the resulting image agrees strongly with gamma-ray images of the galaxy. This is significant because astronomers believe such galactic neutrinos are created. When cosmic rays — fast-moving atomic nuclei produced in high-energy or cataclysmic objects — encounter gas and dust in the galaxy, they should produce both gamma rays and neutrinos. Astronomers had already seen gamma rays they thought formed this way—now the neutrinos that were also supposed to form have been discovered.
With the expected improvements in detector technology in the coming years and the advancement of the machine learning algorithm, this picture will only become clearer. This should allow us to explore hidden features of our galaxy that we have never observed before.
Neilson is thrilled with her team’s discovery and can’t wait to see more. In the 1980s, a supernova explosion in a Milky Way satellite galaxy heralded physicists’ first detection of neutrinos from space. But their detectors weren’t very good. And since then we have not seen such a neutrino event. “We haven’t had another supernova close by since IceCube was built,” says Neilson. “We’re all waiting to see one. Nature needs to cooperate and do its thing.”
Until then, IceCube is waiting under the ice. “Hopefully in another 10 years I can present a NASA-quality picture of the entire universe, not in light but in neutrinos. That’s the goal of my career,” says Neilson.