Odd radio circles in space may come from black holes at their centres

Astronomers have taken clearest images yet of “odd radio circles” – mysterious radio waves a million light years across – and they all seem to have central galaxies containing active supermassive black holes

By Leah Crane

We finally have a sharper image of one of the weirdest phenomena in space. These aptly named odd radio circles, or ORCs, are circles of radio waves that do not seem to emit any radiation in other wavelengths, unlike many other objects visible in radio waves.

A team of researchers led by Ray Norris at the Australian Commonwealth Scientific and Industrial Research Organisation first spotted these strange circles in 2020 using the Australian Square Kilometre Array Pathfinder radio telescope. Astronomers have now definitively spotted five of them, with several more unconfirmed candidates. Now Norris and his colleagues have used the MeerKAT telescope in South Africa to take the sharpest image yet of an ORC.

Odd radio circles are about one million light years across, larger than even the largest spiral galaxies. Of the five confirmed ORCs, astronomers had seen galaxies at the centres of three of them, which hinted that the circles might be formed by some galactic process.

The new observations from MeerKAT have revealed that all ORCs seem to have central galaxies containing active supermassive black holes. That significantly narrows down the options for what ORCs could be. According to Norris, there are now three main options: they could be debris from a huge explosion in their host galaxies, they could come from jets of material spewed out by supermassive black holes, or they could come from residual energy from bursts of star formation.

It will take more observations with more sensitive radio telescopes to figure out which of these explanations is the correct one. The Square Kilometre Array, a huge array of radio telescopes with sections in both Australia and South Africa, is expected to find many more ORCs and help close the book on what they really are once it is fully constructed in 2028.

Journal reference: Monthly Notices of the Royal Astronomical Society, DOI: 10.1093/mnras/stac701

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