Scientists reveal the origin of odd radio circles larger than galaxies

In 2019, the Australian Square Kilometer Array Pathfinder detected several unusual radio wave circles that were so large that they included whole galaxies at their cores. The University of California, San Diego researchers have now figured out what triggered the development of such huge rings. The scientists characterized these circles as huge shells generated by outflowing , possibly caused by explosive events like as supernovae.

As per the official release, the study sheds light on the nature of “starburst galaxies” that have unusually rapid rates of star formation. In a supernova, a dying star emits a large volume of gas into outer space. When numerous stars explode at the same time in close proximity, the strength of such gas outflow exponentially increases.

The combined power of these events can carry the ejected gas beyond the galaxy's boundaries, resulting in outflowing winds at speeds of up to 2,000 kilometers per second. “These galaxies are really interesting,” said Alison Coil, who is chair of the Department of Astronomy and Astrophysics.

Coil further added: “They occur when two big galaxies collide. The merger pushes all the gas into a very small region, which causes an intense burst of star formation. Massive stars burn out quickly and when they die, they expel their gas as outflowing winds.” The scientists calculated a few odd radio circles (ORCs) to be hundreds of kiloparsecs wide(a kiloparsec is equal to 3,260 light years).

Previously, scientists presented many explanations for the origin of these circles, including planetary nebulae and black hole mergers. Extensive observations using another observatory To corroborate their findings, the study team made extensive observations using the W.M. Keck Observatory in Maunakea, Hawaii.

Their focus centered on unraveling the mystery behind ORC 4, which exposed an extensive amount of “highly luminous, heated, compressed gas.” Surprisingly, ORC 4 measurements exceeded what is generally observed in an ordinary galaxy. It was found that stars in the ORC 4’s host 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,” said Coil in the . The researchers also used numerical computer simulations to replicate the size and properties of this ORC. Intriguingly, the models found that outflowing galactic winds last for 200 million years before terminating.

Following the cessation of the wind, a forward moving shock continues to discharge high temperature gas from the galaxy, generating a radio ring. Simultaneously, a reverse shock forces colder gas to return to the galaxy. The simulated chronology closely matched the estimated one billion year 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. And the surrounding gas just outside the galaxy has to be low density, otherwise the shock stalls. These are the two key factors,” explained Coil. “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,” Coil concluded. The findings were published in the journal ..