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.
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…
Read the full article here