Ghost particles from the dark regions of our galaxy

Thanks to the IceCube telescope at the South Pole, scientists were able to reveal the first neutrino image of our galaxy, the Milky Way, earlier this year. The study was published in the journal Science and is based on several years of observations at the research facility in Antarctica.

Olga Botner is one of the researchers behind the article. She has been affiliated with the telescope at the South Pole since 1998 and has devoted a large part of her professional life to studying neutrinos.

“They have been known in nuclear physics since the 1930s, but are very difficult to detect. They have no electrical charge, their mass is tiny and they very rarely collide. That’s why they’re called ‘ghost particles’, because they can pass through a great amount of matter without colliding,” says Botner, Professor of Physics at Uppsala University.

Telescope under the ice

Neutrinos can move freely through the Earth without colliding, for example. They only become visible when they collide. It is precisely this that IceCube has been built to capture. The 5,000 detectors in the telescope are spread evenly through 1 cubic kilometre of ice, approximately two kilometres down in the glacier.

“The detectors register when a neutrino collides with the ice molecules in IceCube, and that’s what makes it possible to see that a neutrino has arrived. Then you can find out where it has come from.”

When the neutrino collides it is destroyed and generates a spray of secondary particles that move in the same direction. By measuring the energy in these particles, scientists can find out how much energy the neutrino had. The scientists also register how the spray moves through the ice and can then calculate backwards and see the direction the neutrino has come from. It is these kinds of calculations that underlie the neutrino image.

“We have used machine learning to produce the image. To put it in very popular terms, you could say that we can now look at the Milky Way through neutrino glasses, we are seeing our galaxy as we were never able to see it before.”

Opens the way for a new type of astronomy

Botner goes on to explain that you can look at a galaxy from different directions. Seen from above, it resembles a spinning top with a distinct core from which spiral arms extend. You can also look at it from the side.

“Often, when we see the Milky Way photographed in optical light, or with radio frequencies, the view is side-on, we see this disc with a central bulge. The neutrino image is viewed from the same angle.”

The satellite images that we usually see of the Milky Way show light fields where the stellar density is high and dark patches where the light does not penetrate. When looking for neutrinos, these dark regions are particularly interesting.

“Neutrinos are often generated in these areas where the light doesn’t penetrate because that’s where there is most matter. If you put your neutrino glasses on, these areas stand out.”

The researchers have succeeded in capturing neutrinos from other galaxies. Botner believes these achievements will pave the way for new approaches to studying the universe.

“These images open the door for neutrino astronomy. Traditionally, astronomy has largely been a matter of studying light of varying wavelengths. We demonstrate here that astronomy can be conducted using other types of radiation.”

Sandra Gunnarsson