Scientists may have finally solved the mystery of cosmic ORCs, or “strange radio circles” as they are officially known, that are 10 times as wide as the Milky Way and large enough to encompass entire galaxies.
A team of astronomers led by University of California San Diego Professor of Astronomy and Astrophysics Alison Coil pointed to strong winds from exploding starbursts, or supernovae, as the cause of the huge shells of gas seen as ORCs in radio waves. The research was announced Wednesday, January 8, at the 243rd meeting of the American Astronomical Society in New Orleans.
ORCs were first detected by the Australian Square Kilometer Array Pathfinder (ASKAP) in 2019 and represented something truly puzzling that astronomers had never seen before.
Coil and his team used the integral field spectrograph at the WM Keck Observatory on the inactive volcano Maunakea on the island of Hawaii to look at the ORC 4 radio circle, finding highly compressed gas within it and stars roughly 6 billion years old. This suggested to the team that these radio circles could be created by multiple stars exploding almost simultaneously in the same galaxy.
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Do cosmic ORCs tell the story of stars from cradle to grave?
When massive stars reach the end of their lives and explode in supernova explosions, vast amounts of stellar material are blown into the surrounding galaxy as powerful winds.
Coil and his colleagues think that if enough supernovae occurred in the same region of space at roughly the same time, these winds could accelerate to speeds of up to 4.5 million miles per hour, about 5,000 times the speed of a bullet fired from a gun.
The question is: What kind of galaxies host so many stars exploding at once?
This can occur in galaxies that experience periods of intense star formation, which astronomers call “starburst” galaxies.
“These galaxies are really interesting,” Coil 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 formed during a period triggered by such rapid star formation coalescence burn their fuel for nuclear fusion at equally rapid rates, explode as supernova explosions, and expel gas as outgoing winds at similar times.
When these winds blow outward from starburst galaxies, they hit slower-moving gases around the galaxy. This interaction creates a shock wave that produces ORCs that can propagate for hundreds of thousands of light years. To put this in context, the Milky Way is about 98,000 light-years across, and a light-year is the distance light travels in one year, about 6 trillion miles (9.7 trillion kilometers).
Solving the mystery of cosmic ORCs
Before the development of this starburst galaxy-driven supernova wind theory, scientists had proposed several other creation mechanisms to explain ORCs. These included the merger of two black holes and the formation of planetary nebulae during supernova explosions that mark the end of a star’s life.
The problem was that ORCs were only seen in radio waves, and this data was not enough to distinguish between possible models of creation.
Suspecting that ORCs might be the result of later phases of the starburst galaxies they were investigating, Coil and his team began studying ORC 4, with its integral field, the first discovered example of these radio shells and visible from the Northern Hemisphere. Spectrograph at the WM Keck Observatory. This revealed to them a large amount of bright, highly compressed gas.
Continuing their detective work and using radio wave data and optical light observations, the team found that the stars in ORC 4 are approximately 6 billion years old; This indicated to them that an intense burst of star formation was occurring at the heart of this ending radio ring. about 1 billion years ago.
To further their research, the team then turned to a computer simulation developed by study co-author Cassandra Lochhaas, an expert on galactic winds at the Harvard & Smithsonian Center for Astrophysics.
The simulation allowed the team to track ORC 4’s evolution over 750 million years, a fraction of its estimated 1 billion year existence.
The simulation, which replicated the size and properties of ORC 4 and took into account the large amount of shocked gas in the galaxy at its center, showed galactic winds flowing outward over a period of about 200 million years.
Even when these winds stopped, the shock wave they ejected continued to move forward, blowing the hot gas out of the galaxy, creating the radio ring surrounding it, and also causing the cooler gas to fall back into the galaxy.
“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 low density; otherwise there will be shock pauses. Those are two key points.” factors,” Coil said. “It turns out that the galaxies we studied have these high mass outflow rates.
“They’re rare, but they do exist. I think this really indicates that ORCs originate from some sort of galactic wind.”
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The finding that ORCs such as ORC 4 are likely the result of outflowing galactic winds means that these radio circles can be used as a proxy to study these strong winds and answer important questions about galactic evolution.
“ORCs provide a way for us to ‘see’ winds through radio data and spectroscopy. This can help us determine how common these extreme outflow galactic winds are and what the wind life cycle is,” Coil said. “They can also help us learn more about galactic evolution: Do all large galaxies go through an ORC phase? Do spiral galaxies turn elliptical when they no longer form stars?
“I think there is a lot we can learn about ORCs and learn from ORCs.”
In addition to being presented at the American Astronomical Society meeting in New Orleans, the team’s research is also detailed in the January 8 issue of the journal Nature.