"Shadow Blaster" galaxy may have sent ghost particle to Earth

A distant star-forming galaxy nicknamed the "Shadow Blaster" may have sent a mysterious cosmic particle toward Earth. Astronomers believe they have traced the particle's origins to 11 billion light-years away, marking a breakthrough in understanding the mysterious neutrinos.

Neutrinos are abundant throughout the universe and have earned the reputation of ghost particles because they have no electrical charge, have little mass and apparently do not interact with other types of matter.

Supernovae, stellar nuclear reactions and the decay of heavy particles can create neutrinos. But tracking exactly where neutrinos come from, when detectors like the IceCube Neutrino Observatory in Antarctica warn of their presence, has proven more difficult for astronomers.

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

And if the source is an object that remains stable in brightness and does not show bursts of activity, identifying the origin of the neutrino seems impossible. But lead author Urata and his team had a stroke of luck, according to the study published June 17 in the journal Nature Astronomy.

A cosmic coincidence illuminated the Shadow Blaster galaxy shortly after the detection of a high-energy neutrino on Earth, suggesting a burst of activity that led researchers directly to the galaxy - and may point to a new way of searching for the origins of ghost particles.

The Shadow Blaster galaxy is behind the bright red galaxy in the center of this image. -
How the Shadow Blaster galaxy got its nickname
In 2021, the IceCube detector, which has sensors installed deep in the Antarctic ice, picked up the presence of a high-energy neutrino - the kind that scientists detect every two to three years, said Erik Blaufuss, a research scientist in the physics department at the University of Maryland. Blaufuss did not participate in the study.

The event that created the neutrino, called IC 210922A, appeared to occur toward the constellation Eridanus, and the observatory issued an alert to the astronomical community.

Scientists performed rapid follow-up observations in different wavelengths of light to search for the particle's point of origin.

However, they were unsuccessful in detecting exploding stars, gamma-ray bursts, X-rays, or visible light components that could be associated with the neutrino.

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

Days after the alert was released, 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 in star formation called JCMT0402-0424.

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

This infographic shows how the gravitational lensing effect works. - Gemini International Observatory/NOIRLab/NSF/AURA/ALMA (ESO/NAOJ/NRAO)/R. Proctor)
The team nicknamed the galaxy the Shadow Blaster because it is filled with dust, making it nearly invisible in optical light, X-rays or gamma rays, Urata said. "Blaster" references the idea that despite its hidden nature, the galaxy could be a powerful source of high-energy particles and neutrinos, he added.

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

The effect of gravitational lensing occurs when a large galaxy in the foreground of an observation magnifies a distant galaxy behind it, functioning 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 more difficult to detect," Urata said.

Possibly a key source of high-energy neutrinos
Dense star-forming regions in galaxies, such as the Shadow Blaster, which forms new stars at an intense rate, may provide the gas, radiation and magnetic field environments that act as 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 department of physics and the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. He did not participate in the study.

During the early days of the universe, 10 billion years ago, there was an intense burst of star formation in galaxies like the Shadow Blaster. Galaxies also formed cosmic rays, the highest-energy particles in the universe, which can create neutrinos.

But establishing the connection between neutrinos and star-forming galaxies has been a difficult task, given that most of these galaxies are distant and faint due to the amount of dust they contain - a key ingredient in star formation. The possibility of observing the interior of the Shadow Blaster with a gravitational lens eased this difficulty, said Urata.

The red galaxy in the foreground refracts light from the more distant Shadow Blaster galaxy. - Gemini International Observatory/NOIRLab/NSF/AURA/ALMA (ESO/NAOJ/NRAO))
Star-forming galaxies, like the Shadow Blaster, could be a key source of high-energy neutrinos. "Our analysis suggests that this population could contribute up to about 20% of the observed diffuse neutrino background as measured by IceCube," said Urata.

Finding the right galaxy in the vicinity of where the neutrino originated could be an accidental coincidence, Vandenbroucke noted. The researchers "estimate the probability of this being an accidental coincidence is about 1%," he said.

"Detecting more such associations between this class of galaxy and high-energy neutrinos is necessary to establish whether they are, in fact, neutrino sources." Scientists also want to know what conditions inside a star-forming galaxy contribute to the creation of neutrinos.

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

"If some of these galaxies are also sources of neutrinos, then neutrinos could offer 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 through gravitational lensing could also enable further study of ghost particles, which remain mysterious despite decades of detection.

"Neutrinos offer a kind of super X-ray vision, allowing us to study phenomena that would otherwise be obscured to our telescopes, in a way analogous to how X-ray machines allow us to see inside people and objects," said Vandenbroucke.

Source: CNN