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Anomalous radio circles in space have mystified astronomers since the cosmic objects were first discovered in 2019. Now, scientists think they may be able to understand the shape of these mysterious celestial structures, and the answer could be provide insight into galactic evolution.
The odd radio circles, also known as ORCs, are so massive that entire galaxies reside in their centers, and the objects span hundreds of thousands of light years.
Our Milky Way galaxy is 30 kiloparsecs across, and one kiloparsec equals 3,260 light years. The odd radio circles measure hundreds of kiloparsecs across. So far, only 11 have been found, some of which are potential ORCs that have not been confirmed, according to researchers.
Astronomers have come up with many theories as to what the space rings could be, including that they are the result of massive cosmic collisions. But a new study suggests that the circles are actually shells sculpted by the powerful galactic winds created when massive stars explode.
Communicate ORCs
The odd radio circles were first spotted by astronomers using the SKA Pathfinder telescope, operated by Australia’s national science agency CSIRO, or the Commonwealth of Scientific and Industrial Research.
The telescope can scan large parts of the sky to detect faint signals, which has enabled scientists to discover unusual objects.
Researchers using the South African Radio Astronomy Observatory’s MeerKAT telescope also took the first image of an ORC, labeled ORC 1, in 2022. (MeerKat is short for Karoo Array Telescope, formerly the Afrikaans word for “more.”) The powerful telescope is sensitive to faint radio light.
The theories came in after the discovery of the odd radio circles: Perhaps they were the throat of a wormhole, the remnants of black hole collisions or powerful jets pumping out energetic particles, according to researchers.
But before the new study, the circles were only observed through radio waves. Despite their massive size, the odd radio circles have not been detected by visible light, infrared telescopes or X-rays.
University of California San Diego astronomy and astronomy professor Alison Coil and her colleagues decided to take a closer look at ORC 4, the first detectable radio oddball from the Earth’s Northern Hemisphere. Coil and her team studied ORC 4 using the WM Keck Observatory on Maunakea, Hawaii, which revealed the presence of heated gas more luminous in visible light than seen in typical galaxies.
The result only prompted more questions.
Missing link
Coil was impressed by the odd radio waves as she and her fellow researchers study massive “constellations of galaxies” that have a high rate of star formation. The galaxies can also quickly drive outflow winds. When massive stars explode, they release gas into interstellar space, or the space between the stars.
When enough stars explode at once, the force from the explosions can drive the gas out of a bursting galaxy at up to 4,473,873 miles per hour (2,000 kilometers per second).
“These galaxies are fascinating,” Coil, lead study author and chair of the University of California San Diego’s astronomy and astrophysics department, said in a statement. “They happen when two large galaxies collide. The merger squeezes all the gas into a very small region, causing an intense burst of star formation. Massive stars flow out quickly and when they die, they release their gas as an outflowing wind.”
Coil and her team thought that the radio rings could be related to constellations of galaxies.
Using visible and infrared light data, Coil’s team calculated that the stars within the galaxy inside ORC 4 are 6 billion years old.
“There was a burst of star formation in this galaxy, but it stopped about a billion years ago,” Coil said.
Then, study co-author Cassandra Lochhaas, a postdoctoral fellow at the Harvard & Smithsonian Center for Astrophysics, ran simulations to recreate the size and properties of the radio ring including the amount of gas they detected with the Keck telescope.
Lochaas’ simulation showed that outflow galactic winds blew for 200 million years before ceasing. Then, the advancing shock wave continued to send hot gas out of the galaxy, creating the radio ring. Meanwhile, a reverse shock forced cooler gas back into the galaxy.
These events took place over an estimated period of 750 million years.
The new research was published in the journal Nature and was presented at the 243rd meeting of the American Astronomical Society in New Orleans on Monday.
“To make this work you need a high mass outflow rate, which means a lot of material is being ejected very quickly. And the surrounding gas just outside the galaxy must be low-density, otherwise the shock will exist. These are the two main factors,” said Coil. “The galaxies we are studying clearly have these high outflow rates. They are rare, but they exist. I really think this points to ORCs coming from some sort of outflow galactic winds.”
Understanding the origins of the odd radio circles also helps astronomers to ultimately understand how the phenomena may have shaped galaxies over time.
“ORCs provide a way for us to ‘see’ the winds through radio data and spectroscopy,” said Coil. “This can help us find out how common these large outflow galactic winds are and what the lifetime of the wind 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 making stars? I think there is a lot we can learn about ORCs and learn from ORCs.”
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