Space mysteries: Odd radio circles may be formed by galactic winds 

In 2019, the Australian Square Kilometer Array Pathfinder (ASKAP) telescope detected something entirely unprecedented in the vast expanse of space: colossal radio wave circles, so large they encapsulated entire galaxies within their centers. 

This discovery, a true space oddity, has spurred intense research and debate. Now, a team led by Professor Alison Coil from the University of California San Diego has had a potential breakthrough in understanding the origins of these enormous rings, known as “odd radio circles” (ORCs).

Discovery of the unknown

Odd radio circles were first observed through the technological ability of ASKAP, allowing astronomers to scan large swathes of the sky at previously unattainable faint limits. 

ORCs are massive, measuring hundreds of kiloparsecs across – dwarfing even the Milky Way galaxy in size. The discovery of these rare rings posed a new challenge for astronomers to understand their origin and nature.

Theoretical breakthrough

Professor Coil and her team have proposed a fascinating theory. They speculate that these circles are shells formed by the powerful galactic winds generated by massive starbursts in galaxies.

“These galaxies are really interesting. They occur when two big galaxies collide. The merger pushes all the gas into a very small region, causing an intense burst of star formation,” said Coil.

Starburst galaxies

Starburst galaxies, characterized by their exceptionally high rate of star formation, play a crucial role in this theory. When massive stars in these galaxies reach the end of their lifespans, they explode as supernovae, ejecting gas at phenomenal speeds. 

The simultaneous explosions can generate outflowing winds strong enough to push this gas out of the galaxy, reaching velocities up to 2,000 kilometers per second.

Studying ORC 4

To further explore this theory, Coil’s team focused on ORC 4, the first odd radio circle observable from the Northern Hemisphere. Utilizing an integral field spectrograph at the W.M. Keck Observatory, the team detected vast amounts of luminous, heated, compressed gas within ORC 4 which far exceeded that of average galaxies. 

Optical and infrared data indicated that the stars within the ORC 4 galaxy are approximately 6 billion years old, with a significant starburst event having occurred a billion years ago.

Outflowing galactic winds 

Cassandra Lochhaas, a postdoctoral fellow specializing in the theoretical aspects of galactic winds, conducted extensive computer simulations to replicate the properties of ORC 4. 

The models showed that outflowing galactic winds blowing for 200 million years could create such a radio ring. 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, suggested the researchers. 

“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.”

Study implications 

While outflowing winds can help astronomers understand odd radio circles, the ORCs can help astronomers understand outflowing winds as well. 

“ORCs provide a way for us to ‘see’ the winds through radio data and spectroscopy,” said Coil. “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 is published in the journal Nature.

Image Credit: © J. English (U. Manitoba)/EMU/MeerKAT/DES(CTIO)

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