Odd Radio Circles: The Remnants of Furious Galactic Winds

It’s not often that — amongst the haphazard jumble of celestial objects — a huge, perfect circle pops out of the data.

That’s just what happened in 2021, as astronomers were examining a sensitive survey of radio emission across the southern sky, conducted with the Australian Square Kilometer Array Pathfinder (ASKAP). They caught three giant rings, each one spanning hundreds of thousands of light-years. “It was incredibly exciting when we discovered the first ORCs,” says Ray Norris (CSIRO and Western Sydney University, Australia), who led the discovery team. “It was clear that nothing like that had been seen before.”

Additional searches through archival data as well as new radio surveys turned up a few more such odd radio circles (ORCs), though they remain rare. These rings turned out to be the edges of gigantic shells of gas expanding outward from a central galaxy. But what happened in those galaxies to generate such colossal shapes?

Alison Coil (University of California, San Diego) and colleagues, publishing in Nature, think they have the answer. They took a closer look at the fourth known ORC using the Keck Cosmic Web Imager, which is designed to detect very faint, extended objects, and turned up a wide swath of shocked gas (as traced by singly ionized oxygen atoms) around the massive central galaxy.

Large galaxies often host some shocked gas due to the activity of newborn and dying stars or a gas-guzzling black hole, but this case is more extreme than most. Though it doesn’t fill in the whole radio circle, the shocked gas around this galaxy spans more than 100,000 light-years. That shocked gas may have come from a long-ago eruption of starbirth.

“The bulk of the light from the stars shows an old stellar population with an age of  some 6 billion years,” Coil explains. “But there appears to also have been a more recent "burst" of star formation, where many stars formed over a relatively brief period of time, and that happened around 1 billion years ago.”

When the massive stars among those stellar newborns died in a volley of supernovae, they would have powered a galactic wind, sending material shooting out of the galaxy and plowing straight into the sparse gas that surrounds it. That material dragged with it magnetic fields, and that’s why we can still see the gas, because particles spiraling around magnetic field lines produce radio waves.

Meanwhile, heated gas left in that expanding shock’s wake fell back in toward the galaxy. It’s cooling as it falls, which is why we see the faint visible light from the radiating oxygen atoms.

“Those winds — especially from a strong starburst — don't last for very long,” Coil adds. “ORCs therefore allow us to ‘see’ the winds after they have stopped blowing.” In fact, the starburst might itself explain why the galaxy since stalled all starbirth — any gas that would have formed stars is long gone.

Norris, who wasn’t involved in the Nature study, agrees with the researchers’ results. His own study of the first known ORC’s detailed structure led to similar conclusions. “What I found interesting about the paper was that they showed that there was clearly something unusual going on in the host galaxy, which fits in well with the rarity of these ORCs,” he says.

He does note, however, that Coil and colleagues also found radio emission coming from the galaxy that is probably associated with the central black hole lurking there. That means the black hole is likely feeding on gas and powering a jet, even if the feeding itself is hidden from view. “Although they may be right about the outflow from the starburst, it is clear from the radio emission from the host that there is an active galactic nucleus there, so that has to be factored into the explanation.,” Norris adds.

ORCs are fascinating in and of themselves, but their existence also has broader implications. They open a window to a part of the universe that we can’t easily see by other means: the gaseous filaments that string between galaxies.

“While we think of galaxies as being isolated islands of stars, they are in fact surrounded by low density gas and are connected on very large scales to the cosmic web; gas is flowing into and out of galaxies all the time,” Coil says. “ORCs can potentially allow us to probe the conditions of the gas far from galaxies which we can't see otherwise.”

These strange radio structures tantalize astronomers, and there’s much work to be done to fully understand how they come about.

“My only regret is that I am semi-retired!” Norris says. “Actually, I would have retired by now . . . but how can you walk away from something like that?”