Uncovering the mystery behind rare radio circuits in the universe

Galactic winds flowing from exploding stars may explain the massive rings.

It's not every day that astronomers say: “What is this?” After all, most of the observed astronomical phenomena are known: stars, planets, black holes, galaxies. But in 2019, the newly completed ASKAP (Australian Square Kilometer Array) telescope captured something no one had ever seen before: radio wave circles so large that they contain entire galaxies at their centers.

While the astrophysics community was trying to determine what these circles were, they also wanted to know Why It was the circles. Now a team led by Allison Coyle, a professor of astronomy and astrophysics at the University of California San Diego, believes they may have found the answer: the circles are shells formed by flowing galactic winds, perhaps from massive exploding stars known as supernovas. Their work is published in nature.

Individual radio circles, like ORC 1 in the image above, are large enough to contain galaxies at their centers and are hundreds of thousands of light-years across. Source: © J. English (U. Manitoba)/EMU/MeerKAT/DES(CTIO)

Cowell and her collaborators have been studying massive “starburst” galaxies that can propel these extremely fast flowing winds. Starburst galaxies have an exceptionally high rate of star formation. When stars die and explode, they expel gas from the star and its surroundings into interstellar space. If enough stars explode close together at the same time, the force of these explosions can push gas out of the galaxy itself and into the flowing wind, which can travel at speeds of up to 2,000 kilometers per second.

“These galaxies are really interesting,” said Cowell, who is also chair of the Department of Astronomy and Astrophysics. “It occurs when two large galaxies collide. The merger pushes all the gas into a very small area, causing an intense explosion of star formation. Massive stars burn up quickly, and when they die they expel their gases in flowing winds.”

Huge, rare and of unknown origin

Technological advances allowed ASKAP to scan large portions of the sky within very faint limits, making individual radio circuits (ORCs) detectable for the first time in 2019. The ORCs were enormous – hundreds of kilometers across, with one kiloparsec equal to 3,260 light-parsecs. years (for reference, milky way The galaxy is about 30 kiloparsecs).

Multiple theories have been proposed to explain the origin of ORCs, including planetary nebulae and… Black hole mergers, but radio data alone cannot distinguish between the theories. Cowell and her collaborators were intrigued and thought it was possible that the radio rings were an evolution from later stages of the starburst galaxies they were studying. They began searching for ORC 4, the first ORC discovered and observable from the Northern Hemisphere.

Simulation of winds driven by stellar explosions at three different time periods, starting at 181 million years ago. The upper half of each image shows the gas temperature, while the lower half shows the radial velocity. Credit: Cassandra Luchas / Space Telescope Science Institute

Until then, ORCs had only been observed through their radio emissions, without any optical data. Cowell's team used an integrated field spectrograph at the WM Keck Observatory on Maunakea, Hawaii, to look at ORC 4, which revealed a huge amount of extremely luminous, hot, compact gas — far more than is seen in the average galaxy.

With more questions than answers, the team gets down to detective work. Using optical and infrared imaging data, they determined that the stars within the ORC 4 galaxy are about 6 billion years old. “There was an explosion of star formation in this galaxy, but it ended about a billion years ago,” Cowell said.

Simulations and conclusions

Cassandra Luchas, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics who specializes in the theoretical side of galactic winds and a co-author of the paper, ran a set of numerical computer simulations to replicate the size and properties of broadband radio. A ring, including a large amount of cold gas shocked into the central galaxy.

Their simulations showed that the flowing galactic winds blow for 200 million years before they stop. When the winds stopped, the forward-moving shock continued to push the high-temperature gas out of the galaxy and created a radio ring, while the reverse shock sent cold gas falling back onto the galaxy. The simulation took more than 750 million years, within the estimated stellar age of 1 billion years for ORC 4.

A computer simulation of a jet galactic wind fired at an initial speed of 450 kilometers per second and a mass flow rate of 200 solar masses per year, which blows gas out of the galaxy for 200 million years into the surrounding galactic medium. The left panel shows the temperature of the gas and the right panel shows the density of the gas. This simulation provides a possible explanation for the origin of individual radio circuits. Credit: Cassandra Luchas / Space Telescope Science Institute

“To make this work, you need a high-mass outflow rate, which means it's ejecting a lot of material very quickly. And the surrounding gas outside the galaxy has to be low-density, otherwise the shock will stop. Those are the two key factors,” Cowell said. “It turns out that the galaxies we've been studying have high-mass flow rates. They're rare, but they do exist. I really think this points to ORCs arising from some sort of flowing galactic wind.”

Not only can flowing winds help astronomers understand ORCs, ORCs can help astronomers understand flowing winds as well. “ORCs give us a way to see winds through radio data and spectroscopy,” Cowell said.

“This can help us determine how common these intensely flowing galactic winds are and what the life cycle of the wind is. They can also help us learn more about galaxy evolution: Do all massive galaxies go through an ORC phase? Do spiral galaxies turn into ellipsoids when they stop forming stars?” ?I think there is a lot we can learn about ORCs and learn from ORCs.

Reference: “Ionized gas extends more than 40 kiloparsecs into a single radio circuit host galaxy” by Alison L. Coyle, Serena Perrotta, David S. N. Rupke, Cassandra Lochhas, and Christy A. Tremonti, Alex Diamond Stanek, Drummond Fielding, and James E. Geach, Ryan C. Hickox, John Moustakas, Gregory H. Rudnick, Paul Sell, and Kelly E. Yen, January 8, 2024, nature.
doi: 10.1038/s41586-023-06752-8