Ghost particles from the dark regions of our galaxy

2023-12-04

A researcher explains: When researchers affiliated with the IceCube telescope revealed the first neutrino image of the Milky Way in the journal Science last summer, it caused a sensation. Professor Olga Botner helps us put on our neutrino glasses and explains what it is we are seeing.

An image showing the Milky Way depicted after locating neutrinos. Looks like a space image with a blue glowing line in the middle.
The neutrino image of the Milky Way above has been created by an artist but is based on observations and measurements at the IceCube telescope. Credit: IceCube Collaboration/U.S. National Science Foundation (Lily Le & Shawn Johnson)/ESO (S. Brunier)

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.

Portrait Olga Botner
Olga Botner, Professor of Physics. Photo: Mikael Wallerstedt

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.”

Figure from the article in Science showing the fields where the highest neutrino density was observed.
The image shows what IceCube has actually observed. The dark areas are the regions where most neutrinos have been observed. The darker it is, the greater the density of neutrinos. Photo from the article “Observation of high-energy neutrinos from the Galactic plane”. The IceCube Collaboration: R. Abbasi et al., Science 380, 6652 (2023).
An image showing the Milky Way depicted after locating neutrinos. Looks like a space image with a blue glowing line in the middle.
The neutrino image is constructed by an artist based on IceCube’s calculations in the image above. Credit: IceCube Collaboration/U.S. National Science Foundation (Lily Le & Shawn Johnson)/ESO (S. Brunier)

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 image shows a bright band of stars covered by black clouds.
The stellar density of our own Milky Way is seen here. The light areas contain many stars. The dark areas have thick clouds of matter. It is in the dark areas that neutrinos often are generated. The image is constructed from observations by the Gaia satellite. Copyright: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: A. Moitinho

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.”

The IceCube telescope's research facility photographed so that you can see both the Milky Way and the northern lights in the background.
The detectors in the IceCube telescope lie deep in the glacier. Photo: Yuya Makino, IceCube/NSF

Find out more

The first neutrino image of our galaxy

”Observation of High-Energy Neutrinos from the Galactic Plane”, Science

Last modified: 2023-08-04