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Strange radio rings in space have been puzzling astronomers since the cosmic objects were first discovered in 2019. Now scientists think they may be able to understand what makes up these mysterious celestial structures, and that the answer could provide insights into galactic evolution.
The peculiar radio circles, also known as ORCs, are so large that entire galaxies lie at their centers, and the objects span hundreds of thousands of light-years.
The diameter of our Milky Way galaxy is 30 kiloparsecs, and one kiloparsec is equal to 3,260 light years. Single radio circles are hundreds of kiloparsecs in diameter. Only 11 have been identified so far, and some of these are potential ORCs that have not yet been confirmed, according to the researchers.
Astronomers have come up with many theories to determine what might form space rings, including that they are the result of massive cosmic collisions. But a new study suggests that the circles are shells shaped by strong galactic winds created when massive stars explode.
Identification of ORCs
Astronomers first detected the strange radio rings using the SKA Pathfinder telescope operated by Australia’s national science agency CSIRO, or Commonwealth Scientific and Industrial Research Organisation.
The telescope can scan large sections of the sky to detect weak signals, allowing scientists to discover unusual objects.
Researchers using the South African Radio Astronomy Observatory’s MeerKAT telescope also captured the first image of an ORC labeled ORC 1 in 2022. (MeerKat stands for Karoo Array Telescope, preceded by the Afrikaans word for “more”). The powerful telescope is sensitive to weak radio light.
After the strange radio rings were discovered, theories arose: Perhaps they were the throats of wormholes, the remnants of black hole collisions, or powerful jets pumping out energetic particles, the researchers hypothesized.
But before the new study, the circles were only observed via radio waves. Despite their enormous size, no visible light, infrared or X-ray telescopes have been able to detect these strange radio rings.
University of California San Diego astronomy and astrophysics professor Alison Coil and her colleagues decided to take a closer look at ORC 4, the first known strange radio ring observable from Earth’s Northern Hemisphere. Coil and his team studied ORC 4 using the WM Keck Observatory in Maunakea, Hawaii; This observation revealed the presence of heated gas brighter in visible light than seen in typical galaxies.
This finding only triggered more questions.
A missing link
Coil is fascinated by strange radio environments because he and his fellow researchers study massive “starburst galaxies” that have high rates of star formation. Galaxies can also drive rapidly outward-flowing winds. When giant stars explode, they release gas into the interstellar space, or the space between stars.
When enough stars explode simultaneously, the force from the explosions can propel gas out of the starburst galaxy at speeds of up to 4,473,873 miles per hour (2,000 kilometers per second).
“These galaxies are really interesting,” Coil, lead author of the study and head of the astronomy and astrophysics department at the University of California, San Diego, said in a statement. “They arise when two large galaxies collide. The merger pushes all the gas into a very small region, causing an intense burst of star formation. “Massive stars burn out rapidly, and when they die they expel their gas outward as wind.”
Coil and his team thought the radio rings might be related to starburst galaxies.
Using visible and infrared light data, Coil’s team calculated that the stars in the galaxy inside ORC 4 are 6 billion years old.
“There was a burst of star formation in this galaxy, but that ended about a billion years ago,” Coil said.
Next, study co-author Cassandra Lochhaas, a postdoctoral researcher at the Harvard & Smithsonian Center for Astrophysics, ran simulations to reconstruct the size and properties of the radio circle, including the amount of gas they detected with the Keck telescope.
Lochhaas’ simulation showed that galactic winds blew for 200 million years before stopping. The forward-moving shock wave then continued to send hot gas out of the galaxy, forming the radio ring. Meanwhile, a reverse shock sent cooler gas back into the galaxy.
These events took place over an estimated 750 million years.
The new research was published in the journal Nature and presented Monday at the 243rd meeting of the American Astronomical Society in New Orleans.
“For this to work, you need a high-mass exit rate, which means a lot of material is expelled very quickly. And the surrounding gas just outside the galaxy needs to be of low density, otherwise the shock would stop. Those are two important factors,” Coil said. “The galaxies we studied turned out to have high mass ejection rates. They are rare but they exist. “I think this really points to ORCs originating from some sort of galactic wind.”
Understanding the origins of strange radio circles also helps astronomers ultimately understand what impact this phenomenon may have in shaping galaxies over time.
“ORCs provide a way for us to ‘see’ winds through radio data and spectroscopy,” Coil said. “This could help us determine how common extreme outflow galactic winds are and what the wind life cycle is. They could also help us learn more about galactic evolution: Do all massive galaxies go through an ORC phase? Spiral galaxies become elliptical when they no longer form stars.” Are they returning? I think there’s a lot we can learn about ORCs and learn from ORCs.”
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