Astronomers have identified a possible new member of one of astronomy’s strangest classes of objects: Odd radio circles (ORCs), enormous ring-like structures visible only at radio wavelengths. The newly discovered source, J1248+4826, appears to be the most compact ORC candidate identified so far, with a ring only about 30,000 parsecs across. The paper was posted to the arXiv preprint server on May 6.
Enigmatic ring
First reported in 2021, odd radio circles are faint ring-shaped radio emissions that are not detected at other wavelengths. They typically have a massive early-type galaxy with hundreds of billions of solar masses at their center, existing somewhere when the universe was roughly 8–11 billion years old. They span hundreds of kiloparsecs across.
Where they come from is still an open question. Leading ideas include remnants of magnetized plasma ejected roughly a billion years ago by a supermassive black hole, a past starburst episode, merging galaxy groups, or large-scale active galactic nuclei-driven outflows.
In the new study, researchers led by M. Polletta of INAF report the discovery of J1248+4826 while inspecting data from the LOFAR Two Meter Sky Survey. It is a radio ring surrounded by a faint diffuse envelope, located very close to a galaxy group as seen when the universe was 11.2 billion years old.
An oddball
The team’s analysis revealed that the ring’s radius is about 9 arcseconds, which corresponds to only 30 kiloparsecs physically. This is well below the previously reported radii of the ring-structure of ORCs, which range from 44 to 365 kiloparsecs. The surrounding diffuse envelope, extending to roughly 100 kiloparsecs, is more in line with the other members of the population.
Despite its compact size, the structure shares many of the same radio properties as previously known ORCs. However, unlike other ORCs that have their host galaxy at their center, this structure’s host galaxy was found not at the center but at the edge.
The team compared J1248+4826 against other classes of diffuse radio emission, including cluster radio halos and mini halos, which have similar sizes and luminosities to ORCs. The key difference is that halos tend to peak in brightness at the center, while ORCs are edge-brightened rings. The ring structure of J1248+4826 fits the ORC picture better than any of these alternatives, although its compact size and offset host make the classification tricky. Researchers say that, alternatively, this oddball could represent a new class of diffuse radio source altogether.
Fossil plasma
Astronomers dug more into what could have formed this odd structure. Radio lobes—extended regions of radio emission on either side of the host galaxy created by energetic jets—viewed at an unusual angle can sometimes mimic a ring, but J1248+4826 shows no sign of a second lobe. They also found no evidence that the structure is powered by an active black hole jet or the fading remains of a radio galaxy.
“The absence of ongoing AGN activity in the host, together with the radio morphology and spectral shape of the diffuse emission, disfavor as possible origin of J1248+4826 a currently active radio galaxy or a classical dying radio galaxy,” researchers write in the paper.
Instead, the researchers found a scenario more fitting: old radio plasma, likely injected by past AGN activity, was re-energized by shock waves driven by galaxy interactions or mergers within the group. The team concludes that the galaxy group’s dynamical activity, such as galaxy interactions, motions of galaxies within the group, or turbulence, among other things, likely played an important role in forming this radio ring.
Astronomers note the need for deeper investigation of J1248+4826 and similar radio sources to distinguish between different formation scenarios. Spatially resolved spectral maps, polarization measurements, X-ray observations, and finally, a bigger sample would help understand their properties and origin better.
“Finally, expanding the sample of ORC-like structures across a wider range of sizes and environments and detailed follow-up studies will be key to assess the heterogeneous nature of these sources and establish their origin and role in the lifecycle of radio plasma in galaxy systems,” the researchers conclude.
If confirmed as an ORC, J1248+4826 would push the known population toward smaller physical scales than before. It would also mean that smaller ORCs are more common than previously thought, but harder to detect.
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Publication details
M. Polletta et al, A Compact Radio Ring with a Diffuse Envelope in LOFAR: Odd Radio Circle or Distinct Phenomenon?, arXiv (2026). DOI: 10.48550/arxiv.2605.05174
Journal information:
arXiv
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Astronomers may have discovered the tiniest odd radio circle (2026, May 21)
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