First ever view of the Milky Way seen through the lens of neutrino particles

Data collected by an observatory in Antarctica has produced our first view of the Milky Way galaxy through the lens of neutrino particles. It’s the first time we’ve seen our galaxy painted with a particle, rather than different wavelengths of light.

The finding, published in Science, offers researchers a new window into the cosmos. Neutrinos are thought to be produced, in part, by high-energy charged particles called cosmic rays colliding with other matter. Due to the limitations of our sensing equipment, there’s still a lot we don’t know about cosmic rays. Therefore, neutrinos are another way to study them.

It has been assumed since ancient times that the Milky Way we see arcing across the night sky is made up of stars like our Sun. In the 18th century it was recognized as a flattened plate of stars that we are observing from within. It’s only been 100 years since we learned that the Milky Way is actually a galaxy, or island universe, one among a hundred billion others.

In 1923, American astronomer Edwin Hubble identified a type of pulsating star called a Cepheid variable in what was then known as the Andromeda nebula (a giant cloud of dust and gas). Thanks to earlier work by Henrietta Swan Leavitt, this provided a measure of the distance from Earth to Andromeda.

This proved that Andromeda is as distant a galaxy as our own, resolving a long-standing debate and completely transforming our idea of ​​our place in the universe.

Opening windows

Later, as new astronomical windows opened on the sky, we saw our galactic home in many different wavelengths of light – in radio waves, various infrared bands, X-rays and gamma rays. Now, we can see our cosmic home in neutrino particles, which have very low mass and interact only very weakly with other matter, hence their nickname ghost particles.

Neutrinos are emitted from our galaxy when cosmic rays collide with interstellar matter. However, neutrinos are also produced by stars such as the Sun, some exploding stars, or supernovae, and probably by most of the high-energy phenomena we observe in the universe such as gamma-ray bursts and quasars. Thus, they can provide us with an unprecedented insight into the highly energetic processes in our galaxy, one that we cannot gain using light alone.

Digital operating module.
A digital operations module, part of the IceCube observatory, is lowered into the ice.
Mark Krasberg, IceCube/NSF, Author provided

The groundbreaking new detection required a rather strange telescope that is buried several kilometers deep in the Antarctic ice sheet, under the South Pole. The IceCube Neutrino Observatory uses a gigatonne of ultra-clear ice under enormous pressure to detect a form of energy called Cherenkov radiation.

This weak radiation is emitted by charged particles which, in ice, can travel faster than light (but not in a vacuum). The particles are created by incoming neutrinos, which come from cosmic ray collisions in the galaxy, striking atoms in the ice.

Cosmic rays are mostly proton particles (these make up the atomic nucleus together with neutrons), along with a few heavy nuclei and electrons. About a century ago, these were found to rain uniformly upon the Earth from all directions. We don’t yet definitively know all of their sources, as their directions of travel are confused by the magnetic fields that exist in the space between the stars.

Deep in the ice

Neutrinos can act as unique tracers of cosmic ray interactions deep within the Milky Way. However, spectral particles are also generated when cosmic rays hit the Earth’s atmosphere. So researchers using the IceCube data needed a way to distinguish between neutrinos of astrophysical origin, those originating from extraterrestrial sources, and those created by cosmic ray collisions within our atmosphere.

The researchers focused on a type of neutrino interaction in ice called a waterfall. These result in roughly spherical showers of light and give researchers a better level of sensitivity to the Milky Way’s astrophysical neutrinos. This is because a cascade provides a better measurement of a neutrino’s energy than other types of interactions, even though they are more difficult to reconstruct.

IceCube Observatory
The IceCube Observatory is located at the South Pole.
Erik Beiser, IceCube/NSF, Author provided

Analysis of ten years of IceCube data using sophisticated machine learning techniques yielded nearly 60,000 neutrino events with an energy exceeding 500 gigaelectron volts (GeV). Of these, only about 7% were of astrophysical origin, while the rest was due to the background source of neutrinos that are generated in the Earth’s atmosphere.

The hypothesis that all neutrino events could be due to cosmic rays hitting the Earth’s atmosphere has been definitively rejected at a level of statistical significance known as 4.5 sigma. In other words, our result has only a one in 150,000 chance of being a fluke.

This is slightly less than the conventional 5 sigma standard for claiming a breakthrough in particle physics. However, such an emission from the Milky Way is predicted on solid astrophysical grounds.

With the upcoming expansion of the IceCube-Gen2 experiment it will be ten times larger, we will capture many more neutrino events and the current blurry image will transform into a detailed view of our galaxy, a view we have never had before.

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