Since the cosmic objects were first spotted in 2019, astronomers have been baffled by strange radio circles in space. Scientists now believe they may know what creates these enigmatic celestial formations, and the explanation may shed light on the history of galaxies.

The structures, known as odd radio circles, or ORCs, span hundreds of thousands of light-years and are so large that entire galaxies live in their centers.

A kiloparsec is equivalent to 3,260 light-years, and our Milky Way galaxy is 30 kiloparsecs across. The peculiar radio rings span hundreds of kiloparsecs. Only 11 have been found thus far, and some of those are perhaps orbiting repeating stars that have not yet been verified, according to researchers.

Numerous hypotheses have been proposed by astronomers to explain the formation of the space rings, one of which is that they are the product of enormous cosmic collisions. However, a recent research proposes that the rings are shells formed by the strong galactic winds that result from the explosion of huge stars.

Identifying ORCs

The strange radio rings were initially noticed by astronomers using the SKA Pathfinder telescope, which is run by the Commonwealth Scientific and Industrial Research Organization, or CSIRO, Australia’s national science organization.

The telescope’s ability to find tiny signals over wide swaths of the sky helped scientists find the odd objects.

In 2022, researchers using the MeerKAT telescope at the South African Radio Astronomy Observatory also obtained the first photograph of an ORC, known as ORC 1. (MeerKat is the Afrikaans word meaning “more,” preceding the abbreviation for the Karoo Array Telescope.) The strong telescope can pick quite little radio signals.

After the strange radio circles were found, theories started to circulate. Some suggested that they might be the throats of wormholes, the remains of black hole collisions, or strong jets emitting energetic particles.

However, the circles have only been seen through radio waves before the recent investigation. The strange radio circles went undetected by visible light, infrared, or X-ray telescopes, despite their enormous size.

Alison Coil, a professor of astronomy and astrophysics at the University of California, San Diego, and her colleagues made the decision to examine ORC 4, the first known odd radio circle visible from the Northern Hemisphere of Earth, in further detail. Using the W.M. Keck Observatory on Maunakea, Hawaii, Coil and her team analyzed ORC 4, which showed the presence of more bright hot gas in visible light than seen in ordinary galaxies.

The discovery simply raised further queries.

An absent piece

Because Coil and her colleagues investigate huge “starburst galaxies,” which have a high rate of star production, she was drawn to the peculiar radio circles. Fast-moving outflowing winds can also be generated by galaxies. Gas is released into interstellar space, or the area between stars, when massive stars explode.

Gas in a starburst galaxy can be forced out at speeds of up to 4,473,873 miles per hour (2,000 kilometers per second) when enough stars explode at once.

“These galaxies are really interesting,” said Coil, lead study author and chair of the University of California San Diego’s department of astronomy and astrophysics, in a statement. “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.”

Coil and her colleagues speculated that the radio rings could have anything to do with starburst galaxies.

Utilizing data from visible and infrared light, Coil’s group determined that the stars in ORC 4’s galaxy are 6 billion years old.

“There was a burst of star formation in this galaxy, but it ended roughly a billion years ago,” Coil said.

Subsequently, postdoctoral fellow at the Harvard & Smithsonian Center for Astrophysics and study coauthor Cassandra Lochhaas performed simulations to replicate the radio circle’s dimensions and composition, taking into account the quantity of gas observed using the Keck telescope.

According to Lochhaas’ simulation, the outflowing galactic winds ceased to blow after 200 million years. The radio circle was then formed by the forward-moving shockwave continuing to expel hot gas from the galaxy. A reverse shock, meantime, returned cooler gas to the galaxy.

The estimated duration of these occurrences was 750 million years.

The new study was presented on Monday at the 243rd meeting of the American Astronomical Society in New Orleans. It was published in the journal Nature.

“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,” Coil said. “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.”

Astronomers can better comprehend the possible effects of strange radio circles on the evolution of galaxies by knowing their beginnings.

“ORCs provide a way for us to ‘see’ the winds through radio data and spectroscopy,” 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.”

Topics #Giant strange radio rings #ORC #space