Astronomers tracing the origin of high-energy neutrino event IC 210922A, detected by the IceCube Neutrino Observatory on September 22, 2021, have identified a distant, dust-obscured star-forming galaxy nicknamed “Shadow Blaster” as its most plausible electromagnetic counterpart, according to a study published in Nature Astronomy.
IceCube Detection and the Search for IC 210922A
At 18:17 UTC on September 22, 2021, the IceCube Neutrino Observatory recorded a pattern of light inside a cubic kilometre of Antarctic ice, identifying it as a high-energy neutrino candidate. Neutrinos carry no electric charge and interact only weakly with ordinary matter, allowing them to travel across the universe without being deflected by magnetic fields. However, that same property makes them exceptionally difficult to trace back to their sources.
According to Erik Blaufuss, a research scientist in the department of physics at the University of Maryland who was not involved in the study, IceCube detects this class of high-energy neutrino every two to three years. Following the alert, observatories launched immediate searches across the region in the Eridanus constellation. Fermi found no significant gamma-ray source, the High-Altitude Water Cherenkov Observatory detected no significant gamma rays, Swift reported no X-ray candidate, and the Zwicky Transient Facility found no visible transient, according to researchers.
JCMT0402−0424 and Submillimetre Observations
Less than two days after the alert, the James Clerk Maxwell Telescope on Mauna Kea began mapping the region with its SCUBA-2 camera, discovering an exceptionally bright source at 850 micrometres that was virtually invisible in ordinary optical images. Subsequent observations by the Submillimeter Array refined the coordinates and linked the object to infrared and radio emission.
Led by Yuji Urata, researchers designated the source JCMT0402−0424. The galaxy is heavily laden with dust that absorbs ultraviolet and visible light from young, massive stars, reradiating the energy at far-infrared and submillimetre wavelengths. The team nicknamed the object “Shadow Blaster”.
Gravitational Lensing Reveals Four Arcs
High-resolution continuum observations from the Atacama Large Millimeter/submillimeter Array (ALMA) transformed the spatial picture by splitting the source into four distinct images arranged around a foreground elliptical galaxy.
The four images are not separate starbursts, but rather distorted views of the same background galaxy created by strong gravitational lensing. The massive foreground galaxy curves spacetime, magnifying and multiplying the background light along multiple paths. Using these four arcs alongside optical and infrared measurements, Urata’s team reconstructed an extended star-forming region roughly 1,700 light-years across.
Spectroscopic Distance and Starburst Engine
ALMA detected several rotational emission lines from carbon monoxide and neutral atomic carbon, yielding a consistent spectroscopic redshift of 2.988. This places the observation roughly 11 billion years in the past, during “cosmic noon,” the broad period when the universe was only a few billion years old and star formation reached its peak.
After accounting for gravitational lensing, the researchers estimate that Shadow Blaster forms hundreds of solar masses of stars each year. The data show no clear evidence of a powerful active galactic nucleus dominating its output, leading researchers to favor an intense, compact starburst packed with dense gas and dust as the main engine.
Did you know? Gravitational lensing acts as a natural cosmic telescope. By bending and magnifying light from background sources, it allows astronomers to study distant objects that would otherwise remain too faint to resolve with current instruments.
Probability and Future Neutrino Astronomy
According to the study’s models, compact-core dusty starbursts at cosmic noon could account for roughly 15 to 20 percent of the diffuse astrophysical neutrino flux across energies from tens of teraelectronvolts to petaelectronvolts.
Frequently Asked Questions
What is the “Shadow Blaster” galaxy?
Shadow Blaster (catalogued as JCMT0402−0424) is a distant, dust-obscured starburst galaxy located approximately 11 billion light-years away, identified as the most plausible electromagnetic counterpart candidate for neutrino event IC 210922A.
Why are neutrinos called ghost particles?
Neutrinos possess no electric charge, carry little mass, and interact only weakly with ordinary matter, allowing them to pass through planets and stars without being deflected by magnetic fields.
How did astronomers find Shadow Blaster?
Following an alert from the IceCube Neutrino Observatory in September 2021, astronomers used the James Clerk Maxwell Telescope and the Submillimeter Array on Mauna Kea to detect the hidden galaxy at submillimetre wavelengths.
What role did gravitational lensing play in the discovery?
A massive foreground elliptical galaxy bent and magnified light from Shadow Blaster, creating four distinct arcs that ALMA used to resolve the distant galaxy’s compact structure.
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