Scientists may have finally solved the mystery of cosmic ORCs, or "strange radio circles" as they are officially called, which are so large they are ten times as wide as the Milky Way and can encompass entire galaxies.
A team of astronomers led by Alison Coil, a professor of astronomy and astrophysics at the University of California, San Diego, has pointed out that powerful winds erupting from outbursts of exploding stars, or supernovae, are the cause of enormous gas shells being released into radio waves. seen as ORCs. The research was unveiled Wednesday (Jan. 8) at the 243rd meeting of the American Astronomical Society in New Orleans.
ORCs were first spotted by the Australian Square Kilometer Array Pathfinder (ASKAP) in 2019 and represented something truly mysterious that astronomers had never seen before.
Coil and her 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 radio circle ORC 4, finding highly compressed gas and stars with a rough age of about 6 billion years. This suggested to the team that these radio circles could form when multiple stars explode almost simultaneously in the same galaxy.
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Do cosmic ORCs tell a story of stars from cradle to grave?
When massive stars reach the end of their lives and erupt in supernova explosions, enormous amounts of stellar material are flung into the surrounding galaxy as powerful winds.
Coil and colleagues think that if enough supernovae occur at about the same time in the same region of space, these winds could be driven to speeds of up to 7.5 million kilometers per hour, about 5,000 times faster than a bullet fired from a gun is fired.
The question is: what kind of galaxies could host so many stars exploding around the same time?
This could happen in galaxies that have experienced periods of intense star formation, which astronomers call "starburst" galaxies.
"These galaxies are really interesting," Coil said in a statement. "They form when two large galaxies collide. The merger pushes all the gas into a very small area, causing an intense burst of star formation."
The massive stars formed during such a merger-induced period of rapid star formation burn their fusion fuel at the same high rates, erupting in supernova explosions and ejecting gas at similar times as outflowing winds.
As these winds blow outward from these starburst galaxies, they hit slower-moving gas around the galaxy. This interaction creates a shock wave that generates ORCs, which can spread over hundreds of thousands of light years. To put this in context, the Milky Way is about 98,000 light-years wide and one light-year is the distance light travels in a year, about 6 trillion miles (9.7 trillion kilometers).
Solving the mystery of cosmic ORCs
Prior to the development of this galaxy-driven supernova wind theory, scientists had proposed several other creation mechanisms to explain ORCs. These include the merger of two black holes and the creation of planetary nebulae during the supernova explosions that mark the end of a star's life.
The problem was that ORCs had only been observed in radio waves, and this data was not enough to distinguish between possible creation models.
Suspecting that ORCs might be the result of later stages of starburst galaxies they had been examining, Coil and her team began studying ORC 4, the first example of these radio bursts discovered to be visible from the Northern Hemisphere, with the integral field spectrograph at the WM Keck Observatory. This revealed to them a huge amount of clear, highly compressed gas.
Continuing their detective work and using radio wave data and optical light observations, the team discovered that the stars in ORC 4 were about 6 billion years old, indicating that a burst of intense star formation had occurred at the heart of this radio circle that ended about 1 billion years ago.
The team then turned to a computer simulation developed by study co-author and galactic wind specialist Cassandra Lochhaas of the Harvard & Smithsonian Center for Astrophysics to continue their research.
The simulation allowed the team to track ORC 4's development over the course of 750 million years, just short of its estimated 1 billion year existence.
The simulation, which replicates the size and properties of ORC 4 and takes into account the enormous amount of shocked gas at the galaxy's heart, showed that galactic winds flowed outward over a period of about 200 million years.
Even when these winds stopped, the shock wave they launched continued to work its way forward, blowing hot gas out of the galaxy, creating the radio ring that surrounds it, and also causing cooling gas to fall back into the galaxy.
'For this to work you need a high mass outflow rate, which means a lot of material is ejected very quickly. And the surrounding gas just outside the Milky Way must be low density for the shock to stop. These are the two most important factors," said Coil. 'It turns out that the galaxies we studied have such high mass outflow rates.
"They're rare, but they do exist. I really think this indicates that ORCs come from some kind of outflowing galactic winds."
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The revelation that ORCs like ORC 4 are likely the result of outflowing galactic winds means that these radio circles could be used as a proxy to study these powerful winds and answer important questions about galactic evolution.
"ORCs give us a way to 'see' the winds through radio data and spectroscopy. This can help us determine how often these extreme outflowing galactic winds occur and what the life cycle of the wind is," Coil said. 'They can also help us learn more about galactic evolution: do all massive galaxies go through an ORC phase? Do spiral galaxies become elliptical when they stop forming stars?
"I think we can learn a lot about ORCs and 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 Jan. 8 edition of the journal Nature.
