Astronomers have traced a single high-energy neutrino called IC 210922A to a galaxy nicknamed Shadow Blaster, a dust-shrouded starburst 11 billion light-years from Earth. The finding, published this month in Nature Astronomy, points to a previously underappreciated source class for the universe’s most elusive particles.
The research team, led by Yuji Urata of MITOS Science Co., LTD. in Taiwan, identified the candidate by combining the IceCube Neutrino Observatory’s 2021 alert with follow-up imaging from ALMA, the James Clerk Maxwell Telescope, the Submillimeter Array, and Gemini North. Shadow Blaster sits inside the neutrino’s localization region and shows no signs of the supermassive black hole that previous neutrino source candidates all share. If the link holds, the galaxy would be the first individual dusty star-forming galaxy tied directly to a high-energy neutrino event.
A Ghost Particle Lands at the South Pole
The chain of evidence began on September 22, 2021, when IceCube, an array of 5,160 light sensors buried inside a cubic kilometer of Antarctic ice, registered IC 210922A. The neutrino carried roughly 750 teraelectronvolts of energy, more than 100 times the maximum reached at our largest particle accelerators. The observatory pinned the neutrino’s origin to a region in the direction of the constellation Eridanus, and within a few hours a global follow-up campaign was searching the patch of sky for an electromagnetic counterpart. Telescopes looked for the bright flash of a gamma-ray burst, a supernova, or an accreting black hole, and found none.
NASA’s Fermi Gamma-ray Space Telescope, the ANTARES neutrino telescope, the Neil Gehrels Swift Observatory, the Zwicky Transient Facility, the High-Altitude Water Cherenkov Observatory, and the DESI Transients Survey all combed the region in the days that followed. The DESI team obtained spectra for 249 galaxies inside the IceCube localization region using spare fibers that were unassigned in the main survey. None of those observations produced a convincing transient.
A couple of days after the alert, Urata and his collaborators turned the James Clerk Maxwell Telescope on Maunakea toward the same patch of sky and picked up an exceptionally bright source at submillimeter wavelengths. The Submillimeter Array on the same summit refined the position. What they found was an extraordinarily luminous galaxy catalogued as JCMT0402-0424 and so obscured by dust that it is nearly invisible in visible light. The team later nicknamed it Shadow Blaster because of that hidden character.
Shadow Blaster at a Glance
- ~750 teraelectronvolts: energy of neutrino IC 210922A detected at the South Pole on September 22, 2021
- ~11 billion light-years: distance from Earth, confirmed by a redshift of 2.988
- Four lensed images: distorted copies of Shadow Blaster produced by a foreground galaxy’s gravity
- ~1,500 light-years: width of the dense, dusty core at the galaxy’s center
- ~1% or lower: probability that an unusually bright submillimeter galaxy would randomly land inside the neutrino’s localization region
A Galaxy Hidden in Dust and Gravity
Shadow Blaster is nearly invisible to optical telescopes. The galaxy is so deeply buried in its own dust that visible-light instruments barely register it, and where ALMA can map it in detail the galaxy shines brightest at submillimeter wavelengths. That selective brightness also explains why no X-ray or optical counterpart was found in the first round of follow-ups.
ALMA’s high-resolution imaging revealed something else. The light from Shadow Blaster does not arrive in one tidy image, since a massive elliptical galaxy in the foreground bends and splits the galaxy’s light into four separate arcs arranged around the lens. The same foreground galaxy amplified Shadow Blaster’s infrared brightness from 2.7 trillion to 33 trillion times the Sun’s luminosity, a magnification that let ALMA resolve structures that would otherwise have been far too faint to study at this distance. The reconstruction showed an extended star-forming region about 1,700 light-years across and an even tighter, unresolved core inside it. Together the lensing and the dust made the galaxy both invisible at first glance and accessible on close inspection.
The full team behind the study, listed on the Nature Astronomy paper on compact dusty starbursts and high-energy neutrinos, draws from MITOS Science, National Central University, Chung Yuan Christian University, the National Astronomical Observatory of Japan, Tohoku University, Fukui University of Technology, the California Institute of Technology, the University of Maryland, and NASA Goddard. Gemini North on Maunakea, one half of the International Gemini Observatory, supplied the optical and infrared data needed to weigh the foreground lens and pin down its distance. The lens model in turn let the team turn gravitational magnification into a precision tool rather than an obstacle.
Once the geometry was in hand, the researchers used ALMA’s spectral lines of carbon monoxide and neutral atomic carbon to establish Shadow Blaster’s redshift at 2.988. That redshift places the galaxy in an era known as Cosmic Noon, a few billion years after the Big Bang when galaxies across the universe were forming stars at peak rates. The 11-billion-year light travel time matches the redshift, putting Shadow Blaster firmly in the era when the universe was most efficient at making stars.
The Telescopes That Built the Case
- IceCube Neutrino Observatory: detected IC 210922A in Antarctic ice in 2021 and set the localization region
- James Clerk Maxwell Telescope: spotted the bright submillimeter source days after the alert
- Submillimeter Array: refined the position of that source on Maunakea
- Gemini North (GMOS and GNIRS): measured the distance and mass of the foreground lensing galaxy
- Atacama Large Millimeter/submillimeter Array (ALMA): resolved the four lensed images, mapped the starburst core, and pinned down the redshift
How a Natural Telescope Made the Hidden Galaxy Visible
Gravitational lensing happens when a massive foreground object bends spacetime and deflects light from a more distant source. The geometry around Shadow Blaster turned that effect into a research tool rather than a source of distortion. The lens split Shadow Blaster into four separate copies of itself and, at the same time, brightened the galaxy enough for ALMA to study its interior in fine detail. No telescope on Earth could have produced images of comparable sharpness without the natural magnification.
“The combined GMOS and GNIRS data helped us measure the distance to the lensing galaxy and determine that it is a massive elliptical galaxy,” Urata said. That distance and mass were the inputs needed to model how strongly the foreground lens magnified and warped the background source. Without those inputs, the team could not have separated the intrinsic shape of Shadow Blaster from the lens’s distortion. The lens’s amplification lifted Shadow Blaster’s infrared luminosity from 2.7 trillion to 33 trillion Suns, a jump of roughly twelvefold.
Lensing also gave the team something rarer than a sharper image, with a falsifiable probability to attach to the discovery. The chance of stumbling onto such an unusually bright submillimeter galaxy by pure coincidence inside IceCube’s localization region is roughly 1% or lower, the team estimates. As described in the ALMA release on the four gravitationally lensed images, the team’s paper calls it the lowest-risk candidate yet identified within the region.
Star Formation, Not a Black Hole, Drives the Engine
Until now, every well-attributed individual high-energy neutrino source has been linked to a supermassive black hole. The blazar TXS 0506+056, the active galaxy Messier 77, and the active galaxy PKS 1424+240 each show the hallmarks of an accreting black hole driving particle acceleration. Tidal disruption events such as AT2019dsg and AT2019fdr have also shown candidate associations. The Milky Way itself emits a diffuse glow of high-energy neutrinos along its disk. None of those source classes matched what ALMA saw in Shadow Blaster.
ALMA’s measurements of molecular gas inside Shadow Blaster show no clear sign of an active galactic nucleus or a bright X-ray core. The gas properties point to an intense, compact episode of star formation instead, with the galaxy producing hundreds of solar masses of new stars each year. Packed into a central region only about 1,500 light-years across, that activity creates the kind of dense, gas-rich environment where cosmic rays can collide with surrounding matter often enough to generate neutrinos.
| Feature | Previous neutrino sources | Shadow Blaster |
|---|---|---|
| Central engine | Supermassive black hole | Intense star formation |
| Representative examples | TXS 0506+056, Messier 77, PKS 1424+240 | JCMT0402-0424 |
| Visible at optical wavelengths | Often yes | Faint, heavily dust-obscured |
| Confirmed individual link to a high-energy neutrino | Yes (blazars, active galaxies) | First candidate from a dusty star-forming galaxy, pending confirmation |
A Possible One-Fifth Share of the Cosmic Neutrino Sky
The IceCube detector has been cataloging high-energy neutrinos since the 1960s, and the sources astronomers have pinned so far cannot account for the diffuse background that fills the sky. The shortfall has long suggested that some major source population is still hiding. Shadow Blaster’s profile, a compact, dust-rich core with no black hole and prodigious star formation, looks like a plausible piece of that missing inventory. If galaxies like it are common in the early universe, their collective neutrino output could fill a measurable slice of the gap.
“Our analysis suggests that this population could contribute up to roughly 20% of the observed diffuse neutrino background measured by IceCube,” Urata said. The ALMA-led population modeling places the figure between roughly 15% and at most around 20%, depending on the model assumptions, for neutrinos with energies from tens of teraelectronvolts up to the petaelectronvolt range. A 15-to-20% contribution would leave plenty of work for other source classes, including the black-hole-powered objects already identified.
The Cosmic Noon setting matters because it lines up with the period when the universe as a whole was most efficient at making stars. Galaxies like Shadow Blaster would have been common roughly 10 billion years ago, churning out stars at peak rates while bathing themselves in dense gas and dust. That same density is what could let them act as cosmic-ray calorimeters, trapping energetic particles long enough for many of them to decay into neutrinos and gamma rays. A confirmation here would turn a single alignment into a window on an entire population.
“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,” Martin Still, program director at the NSF Office of Research Infrastructure, said in the NOIRLab release on Shadow Blaster’s identification. The chain that produced Shadow Blaster’s identification ran from a cubic kilometer of Antarctic ice to Maunakea and the Atacama desert in Chile. Each step fed the next, and no single observatory could have closed the case alone.
Where Outside Astronomers Draw the Line
Independent researchers who were not part of the team have welcomed the new paper while flagging what would still need to happen for the link to harden. The reported coincidence probability is suggestive, not decisive, and the neutrino’s localization region is wide enough that other faint sources could still be hiding inside it. The paper itself stops short of declaring Shadow Blaster the source of IC 210922A and uses language like “most plausible counterpart” rather than a confirmed match.
Jacco Vink of the University of Amsterdam, in comments to Sky & Telescope, called the paper “a good case” for the identification. He explained that the tangled magnetic fields inside a turbulent starburst can trap charged particles long enough for collisions with gas to produce neutrinos, a process that does not require a black hole. The match between theory and observation is encouraging, but Vink’s bottom line was quantitative, with more alignments of this kind needed before the case closes.
Only having many more of these dusty star-forming galaxy-neutrino alignments will clinch the case.
Ralph Wijers, an astronomer at the University of Amsterdam, told Sky & Telescope.
Wijers added that, to him, there is still a reasonable chance that the galaxy simply happens to be an interesting object lying inside the error region and unrelated to IC 210922A. Both outside researchers agreed on what the field now needs, with more individual neutrino events paired with similar dusty starburst counterparts. If several more such pairings emerge, the same low coincidence probability would compound, and the case for an entire population of hidden neutrino sources would harden. Until then, the finding is the strongest candidate counterpart identified in IC 210922A’s region, and it sits squarely on the front line of multi-messenger astronomy.
Frequently Asked Questions
What is a neutrino and why is it called a ghost particle?
A neutrino is an elementary particle with no electric charge and almost no mass. It interacts so rarely with other matter that billions of them pass through a human body every second without leaving a trace, a property that has earned them the “ghost particle” nickname. On rare occasions a high-energy neutrino will collide with an atomic nucleus in a detector and produce a flash of blue Cherenkov light, which is how observatories like IceCube catch them.
What is the IceCube Neutrino Observatory?
IceCube is an array of 5,160 light sensors embedded inside a cubic kilometer of clear Antarctic ice at the South Pole. When a high-energy neutrino collides with the ice, the sensors pick up the faint Cherenkov glow from the resulting particle shower. From those signals, astronomers reconstruct the direction and energy of the incoming neutrino, which is how IC 210922A was traced to a region of sky in the direction of the constellation Eridanus.
How does gravitational lensing work?
Gravitational lensing happens when a massive foreground object bends the path of light coming from a more distant source behind it. In the case of Shadow Blaster, a massive elliptical galaxy between Earth and the starburst acted as a natural magnifying glass, splitting the distant galaxy into four distorted images and brightening its infrared light from 2.7 trillion to 33 trillion Suns. The same effect let ALMA resolve structures in the background galaxy that would otherwise have been too faint to see.
Why does the absence of a black hole matter?
Every well-attributed individual high-energy neutrino source so far has been linked to a supermassive black hole in an active galactic nucleus. Shadow Blaster shows no such signature. If star formation alone can produce these neutrinos, dusty starburst galaxies become a new source class to add to the inventory, and a long-standing shortfall between the neutrinos IceCube detects and the sources astronomers can explain would shrink.
