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A mysterious circle of radio waves has been detected, the fifth odd radio circle (ORC) ever spotted, but its cause is still unclear. It could simply be the side view of a galaxy with an active black hole at its centre, although it might be the result of a supermassive black hole merger.

In 2020, at Western Sydney University, Australia, and his colleagues found four strange circles made from radio waves in space. No one had seen such objects before and the team had a few guesses about what they could be but no firm ideas.

“After we published the paper, I was so sure that someone would come out and say, ‘Actually, you’ve missed this really obvious answer’,” says Norris. But that didn’t happen.

Now, Norris and at the Australia Telescope National Facility in Sydney and their colleagues have found another ORC using the Australian Square Kilometre Array Pathfinder telescope array.

The new radio circle appears to have an elliptical galaxy at its centre, as do two of the previously discovered ORCs. It is also similarly bright and roughly the same size as those two ORCs, with a diameter of about a million light years.

The researchers don’t think it is a coincidence that three of the ORCs they have found have galaxies at their centres, and they suggest the galaxies may be related to the radio waves. The other two ORCs don’t have galaxies within them and may be related to a different phenomenon.

Based on the age of the galaxies, the team says the three ORCs are more than a million years old.

Norris and Koribalski each have a favourite possible source for these ORCs. Norris says he believes the circles are the remnants of a galactic explosion, perhaps from the merging of two supermassive black holes.

“These things are so circular, they really look like a sphere of emission to me,” says Norris. That would account for their size and shape, and explain why they have been so hard to find, he says.

“We know that supermassive black hole mergers should happen in theory, and they should be super rare,” says Norris.

But Koribalski says she isn’t fully convinced that this would cause such a big explosion and the resulting ORC. Instead, she thinks we are seeing the end-on view of a giant, lobed galaxy with a black hole in its middle and radio jets shooting out either end, a bit like a dumb-bell shape with one end pointed towards Earth.

The rare nature of ORCs could be explained by the unlikeliness of a galaxy being completely end on with a telescope we use at any one time, she says.

at the University of Cape Town, South Africa, says that the edge-on galaxy idea seems “a very plausible explanation”. But Norris is less sure that these lobes would really look so round.

Nevertheless, both ideas are still entirely speculative. Koribalski says they haven’t found enough ORCs yet to be sure about either, although they are confident they will have more soon. Norris is also working on a 3D model of his explosion hypothesis to see if it holds up.

“It’s the first time in my career – which has been quite long – where I’ve had a puzzle where you have no idea what it is,” says Norris. “It’s everyone’s dream to have a puzzle like that.”

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