One ORC to rule them all (for now)

Like a blurry, green-blue jellyfish pulsating in the vast cosmic ocean, this image is the best captured yet of a mysterious space phenomenon known as an odd radio circle (ORC).

ORCs were first discovered in 2019 by astronomer Anna Kapinska, who was studying a pilot survey of the Evolutionary Map of the Universe (a radio survey of space), which used data from CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope. Puzzled by the bizarre and spooky phenomenon she was looking at, Kapinska labelled the anomaly with four characters: WTF?

Odd radio circles are so named because they’re large, circular objects which are bright around the edges at radio wavelengths, but which can’t be seen with optical, infrared or X-ray telescopes – and at this stage, astronomers don’t really know what they are. 

And they’re massive – about a million light years across, making them sixteen times larger than our own galaxy. But despite their gargantuan size, the objects are difficult to spot, hiding in plain sight.

Theories abound about the true identity of ORCs, from galactic shockwaves to the open throats of wormholes, but what’s clear is we’ve never seen anything like them before. 

“We know ORCs are rings of faint radio emissions surrounding a galaxy with a highly active black hole at its centre, but we don’t yet know what causes them, or why they are so rare,” says study co-author Ray Norris, of Western Sydney University and CSIRO.

Norris isn’t surprised that we’ve made such exciting and unprecedented discoveries, however, because astronomy – and in particular radio astronomy – is going through a technological revolution, as newer, larger and more precise radio telescope arrays, built upon international cooperation, are being tuned into the heavens.

“ASKAP is fantastic at looking at large areas of sky, going very deep, and this is why they’ve never been seen before,” Norris explains. “Because ASKAP was going to observe the sky in a way it had never been observed before, we knew there was a fair chance we would discover new things.”

To help scientists decode the ORC’s cosmic secrets, the South African Radio Astronomy Observatory’s (SARAO) MeerKAT radio telescope has captured the clearest image of an ORC yet, allowing scientists to tease out more details about these strange objects in a new study published today in Monthly Notices of the Royal Astronomical Society.

“So, we’ve had a really close look at our first ORC,” says Norris. 

“Firstly, we see that there’s a whole lot of structure there that we didn’t know about before; we can see a magnetic field around it, and most importantly we can see that at the centre of this ORC, there’s a little blob of radiation which is bang on top of a galaxy.”

That means the team is now fairly confident that ORCs are actually explosions emanating from the centres of other galaxies – but we still don’t know what causes the explosions.

Norris has two contenders for his favourite theory. One is that such an epic explosion may emanate from the collision of two supermassive black holes in the centre of a galaxy; the massive ring of radio-visible energy, about a million light-years across, would be the remnants of that explosion as it travels out far beyond the width of the galaxy itself. 

The other possibility is that the ORCs are actually starbursts.

“In a starburst, a fair amount of the gas in a galaxy is converted in a very short period into stars,” Norris says. 

“If this happened in our own galaxy, when we looked up at the night sky the whole sky would be blazing with supernovae going off, stars merging, the whole sky would be lit up – except of course we’d all be dead, because all the cosmic rays from all these explosions would have killed us off.”

But starbursts, which involve massive blasts of gas in a particularly active galaxy, could in theory produce massive shockwaves.

“Either of these two models produce this enormous bubble of hot gas coming out from the galaxy.”

So, what’s the next step in unravelling this tantalising space mystery? According to Norris, they’ve just got to keep looking.

“Firstly, we need to find more of them,” he says. “Right now, we’ve only got five of them we’re studying.”

“And the ones we know of, we need to study them more,” he adds. MeerKAT in South Africa is the best telescope for the job, because it penetrates the night sky so deeply. 

The next thing is to try and tease out what ORCs look like in other wavelengths – the wavelengths they’ve so far been invisible in, including visible light, infrared light and X-rays.

“If we can study the galaxy at their centre with optical telescopes, we can see if there are any signs that it was a merger of black holes, or perhaps a starburst.”

But any hopes of glimpsing this would require far bigger, more powerful optical telescopes than we have here in Australia: “so the next thing is to try to use some of the biggest telescopes in the world, for example the European Southern Observatory’s ELT telescope in Chile.”

And the thrill, for many scientists, is in the chase.