For nearly a decade, physicists have watched a steady drizzle of energetic neutrinos rain down on Earth. The IceCube Observatory, buried deep in Antarctic ice, has been catching them since 2013.
What nobody has been able to figure out is where they’re coming from.
A new study published in Physical Review D proposes a likely suspect: Little Red Dots, which were discovered by the James Webb Telescope a few years ago.
“Little Red Dots, or LRDs, are a population of compact and unusually red objects discovered in large numbers in the early Universe,” Riku Kuze, astrophysicist, research fellow at Kyoto University, Japan, and first author of the paper, said to Refractor.
A possible interpretation is that each of these dots hosts a supermassive black hole that aggressively feeds on the surrounding gas.
“They are remarkably luminous for their small apparent sizes, and many of them show broad emission lines. These properties suggest that at least some LRDs contain rapidly growing massive black holes,” said Kuze. “And while the true physical nature remains uncertain, one promising interpretation is that a dense envelope of gas surrounds the central black hole. This picture can explain several of their unusual observational properties.”
One of these unusual properties is that, although some LRDs have been detected in X-rays or radio, most remain faint or undetected at these wavelengths compared to ordinary active galaxies.
“In ordinary active galaxies, an accreting black hole can be recognized through bright X-ray, ultraviolet, radio or gamma-ray emission. In the cocooned-black-hole picture, much of this radiation is absorbed or reprocessed by the surrounding gas before it escapes,” Kuze explains. “This is why the object can appear bright in infrared light while remaining faint in several of the wavelengths used to identify energetic black-hole activity.”
In this study, Kuze and his colleagues posit that while these black holes are hidden inside cocoons of gas, they are anything but quiet. On the contrary, they are extremely active, and this activity may be the origin of some of the high-energy neutrinos that have been raining down on Earth for decades.
The mechanism proposed by the team involves a jet launched close to the black hole that remains embedded within the dense gas envelope. The jet propagates along a lower-density polar region, where particles can be accelerated to extreme energies. These particles interact with the intense surrounding radiation and produce neutrinos.
Neutrinos barely interact with anything, so they slip straight through the surrounding gas that traps everything else. This means LRDs, though hidden from many conventional astronomical observations, could be bright sources of neutrinos, which escape their gaseous cocoons unimpeded.
The team's calculations suggest that LRDs could make a non-negligible contribution to IceCube’s diffuse neutrino signal. “Our study does not establish that LRDs are the dominant origin of the IceCube neutrinos. It shows that this newly discovered population could plausibly contribute a fraction of the diffuse neutrino emission under reasonable physical assumptions,” Kuze explained.
Because LRDs are extremely distant and faint, scientists are unlikely to obtain a direct, definitive detection from any single object in the near future. Instead, the researchers propose looking for an indirect signature: a specific change in the mix of neutrino flavors detected at the highest energies.
If LRDs are responsible for producing these neutrinos, they should leave behind a characteristic pattern in the proportions of electron, muon, and tau neutrinos that reach Earth. Future observatories such as IceCube-Gen2 may be sensitive enough to detect this pattern, providing a way to test the hypothesis even without directly observing the sources themselves.
For now, this study brings us a step closer to understanding the mechanisms behind the universe’s mysterious neutrino rain and the hidden black holes that may be driving it. “I would describe this as an additional step towards understanding the origin of the IceCube signal,” Kuze said.
The study was published in Physical Review D
Source: EurekAlert
Fact-checked by Mike McRae
