Astronomers have traced a high-energy “ghost particle” back to Shadow Blaster, a star-forming galaxy located 11 billion light-years away. That means that this particle, a neutrino, had been travelling to us ever since the 13.8 billion year-old universe was just around 3 billion years old.
The discovery offers the first evidence that star-forming galaxies like Shadow Blaster play a significant role in populating the universe with mysterious high-energy cosmic ghost-neutrinos. These particles get their spooky nickname because, possessing virtually no mass and no electric charge, they pass through matter with little to no interaction while moving at nearly the speed of light. For context, as you read the preceding sentence, over 65 billion neutrinos streamed through every square inch of your body; that’s about 100 billion per square centimeter.
Despite the difficulty associated with detecting such particles, humanity has been spotting neutrinos since the 1960s, but only a few sources of these particles have been identified. Neutrinos are the second most abundant particles in the cosmos after photons, particles of light, and the identified sources are nowhere near enough to account for this abundance. That has prompted the search for other, hidden neutrino sources, especially those which can accelerate neutrinos to high energies. Now that hunt has led to the identification of the incredibly bright Shadow Blaster galaxy, officially designated JCMT0402−0424, which shines in infrared, as a potential neutrino source.
“Shadow Blaster possesses the kind of dense, gas-rich environment that theoretical models have long suggested could efficiently produce high-energy neutrinos,” Yuji Urata of MITOS Science Co., LTD. in Taiwan said in a statement. “If confirmed, Shadow Blaster would be the first-ever individual dusty star-forming galaxy directly linked to a high-energy neutrino event.”
Thus far, no other credible candidates exist as potential sources for this high-energy neutrino, designated IC 210922A.
Chasing ghosts
Astronomers were alerted to the existence of IC 210922A half a decade ago when this high-energy neutrino event was detected by the IceCube Neutrino Observatory located in Antarctica. This set the astronomical community scouring space in the direction of the constellation Eridanus for potential sources for an electromagnetic counterpart to this event with a range of telescopes. This turned up no convincing gamma-ray, X-ray or optical counterpart for the neutrino detection, nor could any gamma-ray burst, supernova, or tidal disruption event (in which a black hole violently shreds a star) be linked with IC 210922A.
Urata and colleagues began their personal search with the James Clerk Maxwell Telescope (JCMT), operated by the East Asian Observatory, and the Submillimeter Array (SMA), discovering Shadow Blaster, a galaxy in the right position and with the right level of brightness to be associated with IC 210922A. The team followed this up with an investigation using the Atacama Large Millimeter/submillimeter Array (ALMA), a collection of 66 radio wave antennas in northern Chile.
The detection of this galaxy was possible because it is strongly gravitationally lensed. Gravitational lensing is a phenomenon that occurs when an object of great mass comes between Earth and a distant background source, curving the fabric of spacetime. As light from the background source navigates this curvature, its path is curved. This results in light from the lensed source arriving at different times to our telescopes, causing it to be amplified.
In the case of Shadow Blaster, before the team could learn anything about this distant galaxy, they had to discover more about the object serving as the intermediate gravitational lens, specifically what type of object it is, its mass, and its distance from us. To do this, they turned to the Gemini North telescope and its Gemini Multi-Object Spectrograph (GMOS) and the Gemini Near-InfraRed Spectrograph (GNIRS) instruments.
With the model of a gravitational lens determined, the team discovered that Shadow Blaster is a galaxy with an extremely compact heart filled with dense clouds of gas and dust that is fueling an intense burst of star formation. A region such as this has long been theorized to serve as a powerful particle accelerator. Because Shadow Blaster lacks a feeding supermassive black hole, this research shows that these regions can still serve as cosmic particle accelerators when they harbor sleeping black holes and in the absence of the powerful jets that erupt from active galactic nuclei (AGNs).
As for the overall population of neutrinos, this research could help account for that too. Intensely star-forming galaxies, or starburst galaxies, are believed to have been prevalent around 10 billion years ago in the early universe. Thus, these galaxies could have been producing a multitude of high-energy neutrinos. Proving that may prove difficult, however, as astronomers don’t have the good fortune to find all of these galaxies lurking behind a gravitational lens, meaning they may be too faint and distant to study.
“Our analysis suggests that this population could contribute up to roughly 20% of the observed diffuse neutrino background measured by IceCube,” Urata concluded,
The team’s research was published on Wednesday (June 17) in the journal Nature Astronomy.