This image shows a close-up of the gravitationally lensed galaxy nicknamed ‘Shadow Blaster’, which astronomers have identified as the likely source of the high-energy neutrino event IC 210922A, detected by the IceCube Neutrino Observatory in 2021. Gravitational lensing occurs when a very massive foreground galaxy bends spacetime, acting as a cosmic magnifying glass that enlarges and distorts the image of a more distant galaxy behind it. In this case, a foreground galaxy, which is not visible in this image, is bending the light of the more distant Shadow Blaster galaxy, creating multiple distorted images of it that appear here as yellow arcs. Credit: NOIRLab/ NSF/ AURA/ ALMA (ESO/ NAOJ/ NRAO) Image Processing: TA Rector (University of Alaska Anchorage/ NSF NOIRLab), D de Martin & M Zamani (NSF NOIRLab). CC BY.
Astronomers have uncovered the strongest evidence yet that distant star-forming galaxies contribute to the production of neutrinos.
Neutrinos are one of the fundamental particles of the Universe. They live a ghostly existence with no electric charge, very little mass, and extremely few interactions with matter.
They are also the most abundant particles with mass in the Universe, and can be created through a variety of processes, such as the decay of heavy particles, nuclear reactions in the Sun, and the explosions of stars.
Instruments on Earth have detected high-energy neutrinos arriving from space since the 1960s, and identifying their origin has been a long-standing challenge in astronomy.
While scientists have identified a small number of nearby neutrino sources, they cannot account for the total amount of neutrinos our instruments measure arriving from across the Universe, referred to as the cosmic neutrino background.
Astronomers, therefore, suspect that other major source populations exist but remain hidden.
In this image, the red foreground galaxy is bending the light of the more distant Shadow Blaster galaxy, creating multiple distorted images of it that appear here as yellow arcs. This composite image was created using data from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Gemini North telescope, one half of the International Gemini Observatory, partly funded by the U.S. National Science Foundation and operated by NSF NOIRLab. Credit: International Gemini Observatory/ NOIRLab/ NSF/ AURA/ ALMA (ESO/NAOJ/NRAO) Image Processing: TA Rector (University of Alaska Anchorage/ NSF NOIRLab), D de Martin & M Zamani (NSF NOIRLab) Acknowledgment: PI: Yuji Urata (MITOS Science Co. Ltd.) CC BY.
Shadow Blaster
In a new study, a team led by Yuji Urata of MITOS Science Co., LTD. in Taiwan presents the analysis of a new neutrino source candidate, an extremely bright galaxy, JCMT0402−0424, nicknamed ‘Shadow Blaster’.
This galaxy is located about 11 billion light-years away, has trillions of times the luminosity of the Sun in the infrared, and may provide the long-sought link between high-energy neutrino production and distant star-forming galaxies.
The discovery was made in part using observations from the Gemini North telescope, the James Clerk Maxwell Telescope (JCMT), and the Submillimeter Array (SMA). All three of these telescopes are located on the summit of Maunakea in Hawai‘i.
In 2021, the NSF IceCube Neutrino Observatory in Antarctica alerted the scientific community to a high-energy neutrino event, dubbed IC 210922A, coming from a region of space in the direction of the constellation Eridanus.
This alert triggered rapid follow-up observations across the electromagnetic spectrum to search for a counterpart signal that, if detected, could help identify the neutrino’s source.
Multiple teams of scientists conducted follow-up observations using a variety of telescopes and instruments.
However, they all reported no convincing gamma-ray, X-ray, or optical counterpart, nor any gamma-ray burst, supernova, or tidal disruption event that could be associated with the alert.
Then, a couple of days after the initial alert, Urata and his team initiated observations with JCMT and SMA and discovered Shadow Blaster, whose location and brightness made it a promising candidate for the source of the signal.
To investigate this galaxy further, the team organised follow-up observations with the Atacama Large Millimeter/submillimeter Array (ALMA), managed for North America by the NSF National Radio Astronomy Observatory, and they discovered that Shadow Blaster is located behind a strong gravitational lens.
The Submillimeter Array at the summit of Maunakea in Hawaiʻi. Credit: NOIRLab/ NSF/ AURA/ LL Christensen. CC BY.
Massive elliptical galaxy
Thanks to this lensing effect, the team would be able to study the internal structure of Shadow Blaster, which would otherwise be too distant and too faint to resolve in such detail.
However, to use the lensing effect correctly and to understand how much the lens amplified the neutrino signal, they first needed to know the distance, nature, and mass distribution of the foreground galaxy.
To decipher these details, they used two powerful instruments on Gemini North: the Gemini Multi-Object Spectrograph (GMOS) and the Gemini Near-InfraRed Spectrograph (GNIRS).
Urata said: “The combined GMOS and GNIRS data helped us measure the distance to the lensing galaxy and determine that it is a massive elliptical galaxy.
“This information was crucial for estimating the lens mass distribution and constructing a model of the gravitational lens.”
Combining the lens model with the ALMA imaging data revealed that the central region of Shadow Blaster contains an extremely compact core that is densely packed with gas and dust and forming new stars at an intense rate.
Theoretical models predict that such an extreme environment can act as a natural particle accelerator, where energetic particles repeatedly collide with gas and produce neutrinos.
Additionally, Shadow Blaster does not display any characteristics of possessing an active black hole. This strongly suggests that high-energy neutrinos can be produced not only by spectacular black-hole jets as scientists have observed in nearby galaxies, but also by the intense, densely packed star formation that is common in very distant galaxies.
Martin Still, programme director at the NSF Office of Research Infrastructure, said: “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.
“By combining signals from particles and light, scientists can explore distant cosmic environments and events in unprecedented detail — revealing phenomena that were once only theoretical.”
The James Clerk Maxwell Telescope located near the summit of Maunakea in Hawai‘i. Credit: William Montgomerie, EAO/ JCMT. CC BY.
Serendipitous location
Around ten billion years ago, the Universe was populated with galaxies like Shadow Blaster that were actively forming stars.
During this epoch, galaxies were theoretically producing large numbers of cosmic rays, which are high-energy streams of particles that can generate neutrinos.
Yet obtaining observational evidence that links an individual neutrino event to such a distant galaxy has been extremely difficult since these galaxies are very far away and often deeply hidden behind thick layers of dust.
Shadow Blaster’s serendipitous location behind a gravitational lens makes finding this observational evidence much easier.
Urata said: “Shadow Blaster possesses the kind of dense, gas-rich environment that theoretical models have long suggested could efficiently produce high-energy neutrinos.
Combined with the absence of any more compelling counterpart despite extensive follow-up searches, Shadow Blaster is the most plausible candidate for the source of IC 210922A.
Urata added: “If confirmed, Shadow Blaster would be the first-ever individual dusty star-forming galaxy directly linked to a high-energy neutrino event.”
Compact star-forming galaxies like Shadow Blaster may be numerous throughout the Universe.
As a population, they may therefore contribute a significant fraction of the high-energy neutrino background that fills the cosmos.
Urata concluded: “Our analysis suggests that this population could contribute up to roughly 20% of the observed diffuse neutrino background measured by IceCube.”
The study is published in Nature Astronomy.
