Data collected by an observatory in Antarctica has produced our first look at the Milky Way galaxy through the lens of neutrino particles. This is the first time we have seen our galaxy “painted” by a single particle, rather than by different wavelengths of light.
The result, published in Science, gives 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. Because of the limitations of our detection equipment, we still don’t know much about cosmic rays. Therefore, neutrinos are another way of studying them.
It has been speculated 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 identified as a flat slab of stars that we view from within. It’s only been 100 years since we learned that the Milky Way is in fact a galaxy, or “island universe”, one of a hundred billion others.
In 1923, American astronomer Edwin Hubble identified a type of pulsating star called the “Cepheid variable” in what was then known as the Andromeda “nebula” (a giant cloud of dust and gas). Thanks to the earlier work of Henrietta Swan Leavitt, it provided a measure of the distance from Earth to Andromeda.
It shows that Andromeda is a distant galaxy like ours, settling a long-standing debate and completely changing our notion of our place in the universe.
Subsequently, as new astronomical windows opened in the sky, we saw our galactic home in many different wavelengths of light — in radio waves, in various infrared bands, in X-rays and in gamma-rays. Today, we can see our cosmic abode 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 perhaps most of the high-energy phenomena we see in the universe such as gamma-ray bursts and quasars. Thus, they can give us an unprecedented view of the energetic processes in our galaxy – a view we cannot get from using light alone.
The new breakthrough detection required a rather unique “telescope” buried several kilometers deep in the Antarctic ice cap, under the South Pole. The IceCube Neutrino Observatory uses a gigatonne of ultra-transparent ice under enormous pressure to detect a type 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 space, that hit atoms in the ice.
Cosmic rays are mainly proton particles (these make up the atomic nucleus along with neutrons), along with some heavy nuclei and electrons. About a century ago, they were discovered raining down on the Earth evenly from all directions. We don’t yet know all their sources for sure, because their directions of travel are constrained by the magnetic fields that exist in interstellar space.
Deep in the ice
Neutrinos can act as unique tracers of cosmic ray interactions in the depths of the Milky Way. However, ghostly particles are also formed when cosmic rays strike the Earth’s atmosphere. So researchers using the IceCube data needed a way to distinguish between neutrinos of “astrophysical” origin – those coming from extraterrestrial sources – and those created from cosmic ray collisions in within our environment.
The researchers focused on a type of neutrino interaction with ice called a cascade. This results in a nearly spherical light spill and gives researchers a better level of sensitivity to astrophysical neutrinos from the Milky Way. This is because a cascade provides a better measurement of the energy of a neutrino than other types of interactions, even though they are more difficult to reconstruct.
Analysis of ten years of IceCube data using sophisticated machine learning techniques yielded nearly 60,000 neutrino events with energies greater than 500 gigaelectronvolts (GeV). Of these, only about 7% are of astrophysical origin, and the rest are due to “background” sources of neutrinos generated in the Earth’s atmosphere.
The hypothesis that all neutrino events could be due to cosmic rays hitting the Earth’s atmosphere was conclusively rejected at a level of statistical significance known as 4.5 sigma. Put another way, our result has about a 1 in 150,000 chance of being a fluke.
This falls somewhat short of the conventional 5 sigma criteria for claiming a discovery in particle physics. However, such an emission from the Milky Way is expected on sound astrophysical grounds.
With the upcoming enlargement of the experiment – IceCube-Gen2 will be ten times bigger – we will get more neutrino events and the currently blurry picture will become a detailed view of our galaxy, one that we have never had before.
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