It's not often that space throws something completely new at us, but in 2019, astronomers discovered a completely unknown phenomenon they call strange radio circles (ORCs). Now, more data may reveal how these rare objects form.
All you need to know about ORCs is in the name: They are round clusters of radio emitters, and they're strange because the reasons for their formation are unclear. The first few objects were discovered in data from the Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope, and it was initially unclear whether they were small objects within the Milky Way or giant objects farther out.
ORCs are very faint and invisible to infrared and optical wavelengths, making them difficult to study, but more information emerging in 2022 is starting to help determine their identities. The latter scenario turned out to be true - the ORC is massive, millions of light-years across, and centered on certain galaxies. It turns out that this may be the key to understanding them. In this new study, astronomers have found almost literal ironclad evidence linking ORCs to "Starburst" galaxies.
Alison Coil, first author of the study, said: "These galaxies are very interesting. They appear when two large galaxies collide. The merger pushes all the gas into a very small area, causing a violent burst of star formation. Massive stars burn up quickly and when they die, they expel gas in the form of outflow winds."
Of course, as stars are born, stars also die, so these galaxies will experience a large number of supernovae at the same time over a period of time. Because so many stars are exploding nearby at the same time, the gas they eject is accelerated into winds that flow out of the galaxy at 2,000 kilometers (1,240 miles) per second. The team suspects ORC may be a later stage in this process.
To find out, they looked at galaxies at the center of an ORC using optical and infrared light. Sure enough, there appeared to be a large amount of bright, hot, compressed gas at its center. Based on these data, they determined that the stars in the galaxy are about 6 billion years old, and that the period of crazy stellar explosions ended about 1 billion years ago.
Next, the team ran simulations that calculated the properties of the galaxy, its stars, and the surrounding ORC. The results showed that the outflow winds blew for about 200 million years and then stopped. Then one shock wave rippled out, pushing the hotter gas farther out, creating a radio loop, while another shock wave moved in the opposite direction, pushing the cooler gas back into the galaxy. The entire process took about 750 million years, which the team said is consistent with their original estimate of the age of the structure.
"To achieve that, you need a high-mass outflow rate, meaning it's ejecting a lot of material very quickly," Cole said. "The density of the surrounding gas outside the Milky Way has to be very low, otherwise the impact would stall. Those are two key factors. It turns out that the galaxies we've been studying all have these high-mass outflow rates. They're rare, but they do exist. I really think this suggests that the ORC originates from some kind of outflowing galactic wind."
Then these radio circles might not seem so strange anymore. But the team says we still have a lot to learn about them, and they can teach us about galaxies in general.
"They can also help us learn more about galaxy evolution: Do all massive galaxies go through an ORC phase? When spiral galaxies stop forming stars, do they turn into elliptical galaxies? I think we can learn a lot about ORC from ORC," Cole said. "In the meantime, we are excitedly waiting for the next piece of the cosmic puzzle to emerge from the depths of space."
The research was published in the journal Nature.