Astronomers trace a ghostly cosmic particle to distant ‘Shadow Blaster’ galaxy

By Ashley Strickland, CNN

(CNN) — A distant star-forming galaxy nicknamed the “Shadow Blaster” could have sent a ghostly cosmic particle hurtling toward Earth.

Astronomers believe they have traced the particle‘s origin to 11 billion light-years away, marking a step forward in understanding the mysterious neutrinos.

Neutrinos are abundant across the universe, earning a reputation as ghost particles because they possess no electric charge, have little mass and don’t seem to interact with other types of matter.

Supernovae, stellar nuclear reactions and the breakdown of heavy particles can create neutrinos. But tracing exactly where neutrinos come from when detectors such as Antarctica’s IceCube Neutrino Observatory alert their presence has proven more difficult for astronomers.

“They rarely interact with matter, which is why they can travel across the universe almost undisturbed,” said Dr. Yuji Urata, researcher at Taiwan-based astronomical research firm MITOS Science Co. Ltd. “Even when IceCube detects a high-energy neutrino, the position on the sky often has an uncertainty region that is much larger than the size of a galaxy.”

And if the source is an object that remains steady in brightness and doesn’t flare with activity, zeroing in on the neutrino’s origin feels impossible.

But lead author Urata and his team encountered a stroke of luck, according to their study published June 17 in the journal Nature Astronomy.

A cosmic coincidence brightened the Shadow Blaster galaxy shortly after the detection of a high-energy neutrino on Earth, suggesting a flare of activity that led the researchers right to the galaxy — and could point to a new way to search for the origins of ghost particles.

How Shadow Blaster galaxy got its nickname

In 2021, the IceCube detector, which has sensors embedded deep in the Antarctic ice, picked up the presence of a high-energy neutrino — the kind scientists detect every two to three years, said Erik Blaufuss, research scientist in the department of physics at the University of Maryland. Blaufuss was not involved in the study.

The event that created the neutrino, named IC 210922A, appeared to occur in the direction of the Eridanus constellation, and the observatory released an alert to the astronomy community. Scientists carried out quick follow-up observations across different wavelengths of light to seek out the particle’s origin point.

They were unsuccessful, however, in detecting any exploding stars, gamma-ray bursts, X-rays or visible light components that might be associated with the neutrino.

“Neutrinos alone tell us that something energetic happened somewhere in the sky, but they usually do not tell us exactly what the source is, how far away it is, or what kind of object produced them,” Urata wrote in an email. “To answer those questions, we need light: radio, submillimeter, infrared, optical, X-ray and gamma-ray observations.”

Days after the alert’s release, Urata and his colleagues carried out observations with the East Asian Observatory’s James Clerk Maxwell Telescope as well as the Submillimeter Array, both located near the summit of Mauna Kea in Hawaii. They discovered a galaxy rich with star formation called JCMT0402−0424.

The galaxy had trillions of times the luminosity of our sun in infrared light, and it was in the right location to be potentially connected to the neutrino.

The team nicknamed the galaxy Shadow Blaster, because it is filled with dust, making it nearly invisible in optical light, X-rays or gamma rays, Urata said. Blaster refers to the idea that despite its hidden nature, the galaxy may be a powerful source of high-energy particles and neutrinos, he added.

When the researchers carried out additional follow-up observations using the Atacama Large Millimeter/submillimeter Array in Chile, they realized that Shadow Blaster was located behind a gravitational lens.

Gravitational lensing occurs when a large galaxy in the foreground of an observation enlarges a distant galaxy behind it, acting like a cosmic magnifying glass.

“This lensing effect magnified the galaxy and allowed us to study a hidden, compact star-forming region that would otherwise have been much harder to detect,” Urata said.

Possibly a key source of high-energy neutrinos

Dense stellar nurseries in galaxies, such as the one in Shadow Blaster that forms new stars at a powerful rate, can provide the gas, radiation and magnetic environments that act like particle accelerators to produce neutrinos, he added.

“Star-forming galaxies are galaxies that produce many stars, some of which are massive and burn out quickly, exploding in supernovae, likely accelerating cosmic rays in the process,” said Justin Vandenbroucke, professor in the physics department and the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. He was not involved in the study.

During the early days of the universe 10 billion years ago, there was an intense burst of star formation across galaxies such as Shadow Blaster. The galaxies also formed cosmic rays, the most highly energetic particles in the universe, which can create neutrinos.

But making the connection between neutrinos and star-forming galaxies has been a difficult task given that most of the galaxies are distant and faint due to the amount of dust they contain — a key ingredient in the formation of stars. Being able to peer inside Shadow Blaster with a gravitational lens has eased that difficulty, Urata said.

Star-forming galaxies such as Shadow Blaster could be a key source of high-energy neutrinos.

“Our analysis suggests that this population could contribute up to roughly 20% of the observed diffuse neutrino background measured by IceCube,” Urata said.

Finding the right galaxy in the vicinity of where the neutrino came from could be an accidental coincidence, Vandenbroucke noted.

The researchers “estimate the probability of it being an accidental coincidence to be about 1%,” he said. “Detecting more such associations between this type of galaxy and high-energy neutrinos is necessary to establish whether they are indeed neutrino sources.”

Scientists also want to know what conditions within a star-forming galaxy contribute to the creation of neutrinos.

Observatories such as ALMA and the James Webb Space Telescope are changing how astronomers study distant, dusty, massive galaxies, Urata said.

“If some of these galaxies are also neutrino sources, then neutrinos may provide a completely new way to study how galaxies formed stars, built magnetic fields, and accelerated cosmic rays when the universe was young,” he added.

The study will motivate the search for deeper associations between neutrinos and potential sources going forward, Blaufuss noted.

Finding neutrinos using gravitational lenses could also enable a deeper study of the ghost particles, which still prove mysterious despite their detection for decades.

“Neutrinos provide a kind of super X-ray vision, enabling us to study phenomena that are otherwise obscured from our telescopes, analogous to how X-ray machines enable us to see inside people and objects,” Vandenbroucke said.

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