Odd radio circles (ORCs), a class of extragalactic astronomical sources discovered in 2019, are in fact shells formed by outflowing galactic winds, possibly from massive supernovae, according to new research.
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ASKAP radio continuum image of ORC 1 (contours) overlaid onto a DES 3-color composite image. Two galaxies of interest: ‘C’ lies near the center of ORC 1 and ‘S’ coincides with the southern radio peak. Image credit: Norris et al, arXiv: 2006.14805.
The first three ORCs were discovered in the Evolutionary Map of the Universe Pilot Survey using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope.
A fourth ORC, named ORC4, was discovered in archival data taken with the Giant Meterwave Radio Telescope, and additional ORCs were discovered in later ASKAP and MeerKAT data.
These sources were enormous — hundreds of kiloparsecs across, where a kiloparsec is equal to 3,260 light-years.
Multiple theories were proposed to explain their origin, including planetary nebulae and black hole mergers, but radio data alone could not discriminate between the theories.
University of California San Diego’s Professor Alison Coil and colleagues were intrigued and thought it was possible the radio rings were a development from the later stages of the starburst galaxies they had been studying.
Up until then, ORCs had only been observed through their radio emissions, without any optical data.
The astronomers used an integral field spectrograph at the W.M. Keck Observatory on Maunakea, Hawaii, to look at ORC 4, which revealed a tremendous amount of highly luminous, heated, compressed gas — far more than is seen in the average galaxy.
With more questions than answers, the team got down to detective work.
Using optical and infrared imaging data, they determined the stars inside ORC 4 galaxy were around 6 billion years old.
“There was a burst of star formation in this galaxy, but it ended roughly a billion years ago,” Professor Coil said.
The authors also ran a suite of numerical computer simulations to replicate the size and properties of the large-scale radio ring, including the large amount of shocked, cool gas in the central galaxy.
The simulations showed outflowing galactic winds blowing for 200 million years before they shut off.
When the wind stopped, a forward-moving shock continued to propel high-temperature gas out of the galaxy and created a radio ring, while a reverse shock sent cooler gas falling back onto the galaxy.
The simulation played out over 750 million years — within the ballpark of the estimated one-billion-year stellar age of ORC 4.
“To make this work you need a high-mass outflow rate, meaning it’s ejecting a lot of material very quickly,” Professor Coil said.
“And the surrounding gas just outside the galaxy has to be low density, otherwise the shock stalls. These are the two key factors.”
“It turns out the galaxies we’ve been studying have these high-mass outflow rates. They’re rare, but they do exist. I really do think this points to ORCs originating from some kind of outflowing galactic winds.”
Not only can outflowing winds help astronomers understand ORCs, but ORCs can help astronomers understand outflowing winds as well.
“ORCs provide a way for us to ‘see’ the winds through radio data and spectroscopy,” Professor Coil said.
“This can help us determine how common these extreme outflowing galactic winds are and what the wind life cycle is.”
“They can also help us learn more about galactic evolution: do all massive galaxies go through an ORC phase?”
“Do spiral galaxies turn elliptical when they are no longer forming stars? I think there is a lot we can learn about ORCs and learn from ORCs.”
The study was published in the journal Nature.
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A.L. Coil et al. Ionized gas extends over 40 kpc in an odd radio circle host galaxy. Nature, published January 8, 2024; doi: 10.1038/s41586-023-06752-8
