Odd Radio Circles (ORCs) were discovered by accident in 2019. They are circular regions of faint, edge-brightened radio emissions that are invisible at optical, infrared, ultraviolet and X-ray wavelengths.
Some have a galaxy at the centre while others don’t, but they’re all characterised by their large size, many times bigger than a typical galaxy. Some ORCs seem to have a double-ring structure whereas others are single. Some also have internal arc-like structures that appear to be associated with galaxies embedded within the bubble of radio emission.
Objects with a high degree of spherical symmetry are common in the Universe, but ORCs appear to be different from all of those, leading astronomers to declare them a new class of object.
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ORCs may be a form of spherical shock wave created by, for example, fast radio bursts, gamma-ray bursts or neutron star mergers. If so, they would have to be extremely ancient to have expanded to such large sizes.
Alternatively, they may be associated with the jets of material coming from the central regions of radio galaxies – but, again, their large size, and the fact that not all of them have central objects, are difficult to explain.
A promising idea is that ORCs are formed by the merger of two supermassive black holes in a central galaxy. The available data is also consistent with a shell caused by a ‘shock termination’ from a wind of energetic particles from a central ‘starburst’ galaxy.
Another suggestion is that ORCs are the throats of ‘wormholes’ – theoretical passages through space-time. But astronomers are yet to reach agreement on the true nature of ORCs.
This article is an answer to the question (asked by Brendan Owens, Bradford) 'What are odd radio circles?'
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