Astronomers trace ghost particle to a distant star-forming galaxy

(CN) — Every second, trillions of tiny particles called neutrinos pass through your body unnoticed. They carry no electric charge, have almost no mass and rarely interact with anything around them.

Scientists have been detecting them for decades, but where the most powerful ones come from has remained a mystery.

Now, a team of astronomers may have found an answer.

In a study published Wednesday in the journal Nature Astronomy, researchers led by Yuji Urata of MITOS Science Co. Ltd. in Taiwan say a distant galaxy nicknamed “Shadow Blaster” is the strongest candidate yet for the source of a high-energy neutrino detected in 2021.

The galaxy lies about 11 billion light-years from Earth. If the link holds up, it would be the first time a star-forming galaxy has been directly connected to a high-energy neutrino event.

In 2021, the IceCube Neutrino Observatory in Antarctica detected a high-energy neutrino known as IC 210922A. IceCube alerted the scientific community, and multiple teams scrambled to find a source, scanning the region of sky the neutrino appeared to come from using gamma-ray, X-ray and optical telescopes. None found a convincing explanation.

A few days later, Urata’s team pointed two telescopes on the summit of Maunakea in Hawaii at the same patch of sky and spotted Shadow Blaster. Its position and unusual brightness immediately caught the team’s attention.

Shadow Blaster is one of the brightest known star-forming galaxies in the universe, radiating roughly 2.7 trillion times as much infrared light as the sun. Astronomers found no evidence that an active black hole is powering that output. Instead, they say the energy comes from an intense burst of star formation packed into a compact, dust-filled core.

That environment may be exactly what neutrino theorists have been looking for. Models suggest that high-energy particles can become trapped inside dense clouds of gas and dust, colliding repeatedly and producing neutrinos before escaping into space.

“Shadow Blaster possesses the kind of dense, gas-rich environment that theoretical models have long suggested could efficiently produce high-energy neutrinos,” Urata said. “If confirmed, Shadow Blaster would be the first-ever individual dusty star-forming galaxy directly linked to a high-energy neutrino event.”

Studying Shadow Blaster in detail required a lucky break.

The galaxy sits behind a massive elliptical galaxy whose gravity bends and magnifies light from behind it, a phenomenon known as gravitational lensing. The effect boosted Shadow Blaster’s apparent brightness to about 33 trillion times that of the sun, making it easier to study in detail.

To take advantage of that magnification, the team had to understand the foreground galaxy itself, measuring its distance, mass and structure. They did that using two instruments on the Gemini North telescope on Maunakea.

“The combined GMOS and GNIRS data helped us measure the distance to the lensing galaxy and determine that it is a massive elliptical galaxy,” Urata said. “This information was crucial for estimating the lens mass distribution and constructing a model of the gravitational lens.”

With that model in hand, the team used the Atacama Large Millimeter/submillimeter Array in Chile to peer into Shadow Blaster’s core and confirm just how compact and dense it is.

Shadow Blaster may also explain where many of the universe’s high-energy neutrinos come from. Around 10 billion years ago, the universe was full of galaxies like it, all forming stars at a furious pace.

Scientists have long suspected these galaxies were producing enormous numbers of neutrinos, but finding direct evidence has been difficult because they are so distant and cloaked in dust.

“This breakthrough shows how particle detectors and telescopes become far more impactful when they work together, opening a powerful multi-messenger window on the universe,” said Martin Still, program director at the NSF Office of Research Infrastructure.

If the researchers are correct, galaxies like Shadow Blaster may account for a substantial share of the high-energy neutrinos arriving at Earth from across the cosmos. The team suggests they could produce roughly one-fifth of the diffuse neutrino background measured by IceCube.

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