First neutrino observation of our galaxy

Until now, the Milky Way had been analyzed with visible and invisible light, such as X-rays and radio waves, but the IceCube experiment located in Antarctica has observed it with something that is not light, but particles: high-energy neutrinos from from the galactic plane.

In places without light pollution, we can see the Milky Way as a diffuse band of stars on the horizon. The astronomical community also observes and studies it in detail at different wavelengths, but from now on it will have a new type of ‘lens’: neutrinos.

For the first time, the IceCube neutrino observatory, a gigantic one cubic kilometer detector built under the Amundsen-Scott South Pole Station, has produced an image of our galaxy using these tiny, ghostly astronomical messengers.

The members of this international collaboration, made up of more than 350 scientists, present evidence of the emission of high-energy neutrinos from our galaxy in the journal Science.

“Neutrinos are subatomic particles, as are electrons. However, they are special in that they interact only through the weak force. Just as light can pass through the glass of a window without difficulty, neutrinos can pass through everything, including planet Earth, which is why they are so difficult to detect”, explains the IceCube spokesperson, Ignacio Taboada, professor of Physics, to SINC. at the Georgia Institute of Technology (USA).

“That’s why IceCube is so big, to be able to observe the few neutrinos that do interact,” he continues. And regarding the fact that they are of “high energy”, it is in comparison with other neutrinos, such as those produced by the Sun, which have energies a million times lower.”

Taboada underlines the importance of this finding: “This is the first time that the Milky Way has been observed with something other than light: neutrinos. Visible and invisible light (radio, microwave, infrared, X-rays, gamma rays) have been used extensively to study our galaxy, but neutrinos are not light. And by studying in different ways, you learn new things.”

IceCube Principal Investigator Francis Halzen, Professor of Physics at the University of Wisconsin-Madison, adds: “What is intriguing is that, unlike light of any wavelength, in the case of neutrinos, the universe eclipses nearby sources in our own galaxy.”

The IceCube team had already detected high-energy neutrinos of extragalactic origin, such as those coming from the nearby galaxy NGC1068, and they assume that the same could happen in other more distant ones. But what happens in our Milky Way?

Gamma-ray observations show bright emissions from within the galactic plane, and since gamma rays and neutrinos are thought to be produced by the same astrophysical processes, that plane was the expected site of neutrino emission, as it has been.

The demonstration has been carried out thanks to machine learning artificial intelligence techniques, using data recorded (about 60,000 neutrinos) over 10 years by the IceCube observatory in Antarctica.

The researchers have presented the first statistically robust evidence for high-energy neutrino emission from the galactic plane, with results consistent with the expected distribution and interactions of cosmic rays within our galaxy.

“We detect neutrinos from our own galaxy by studying their direction and energy”, explains Taboada, “there is an excess of these high-energy particles that point approximately in the direction of the plane of the galaxy, and especially towards the galactic center”.

But where exactly do they come from? “It is not possible to know for sure what produces these neutrinos, since we have observed the Milky Way as a whole”, the professor replies, “although there are two reasonable possibilities and probably both occur, but we do not know which is more important

On the one hand, “neutrinos can be produced by cosmic ray sources in our galaxy: a collection of point sources, such as a star, of neutrinos,” he clarifies. But these cosmic rays, which have an electrical charge, propagate through the galaxy and when they collide with gas, stellar dust, etc., they produce more neutrinos.”

“That would result in our galaxy shining diffusely everywhere, but more intensely towards the center,” says Taboada, who anticipates that now “the next step is to identify the specific sources within the galaxy.” That and other challenges will be addressed in the following analyzes provided by IceCube.

Another member of the collaboration, Naoko Kurahashi Neilson, Professor of Physics at Drexel University (USA), concludes: “Observing our own galaxy for the first time using particles instead of light is a big step. As neutrino astronomy evolves, we will get a new lens with which to observe the universe.