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

Astronomers have taken the 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

Space 22 March 2022

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 don’t seem to emit any radiation in other wavelengths, unlike many other objects detectable in radio frequencies.

A team 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 seen 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 1 million light years across, larger than even the biggest 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.

ORC1 image from MeerKAT_Credit J. English, EMU, MeerKAT, DES

An image of an odd radio circle from the MeerKAT telescope in South Africa

J. English, EMU, MeerKAT, DES

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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 alternatives: 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 is correct. The Square Kilometre Array, a huge assembly of radio telescopes with sections in 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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