Galactic winds flowing from exploding stars may explain the giant rings.
It’s not every day that an astronomer says, “What is that?” After all, most of the astronomical phenomena observed are known: stars, planets, black holes, galaxies. But the newly completed ASKAP (Australian Square Kilometer Array Pathfinder) telescope in 2019 captured something no one had ever seen before. It was a circle of radio waves so large that it contained an entire galaxy in the center.
While trying to determine what these circles were, the astrophysics community also wanted to know. why The circle was like that. Now, a team led by Alison Coyle, a professor of astronomy and astrophysics at the University of California, San Diego, believes they may have found the answer. The circle is probably a shell formed by galactic winds flowing out from a giant exploding star known as a supernova. Their works are Nature.
Coyle and his collaborators have been studying giant “starburst” galaxies that generate these ultrafast outflow winds. Starburst galaxies have a very high rate of star formation. When a star dies and explodes, gas from the star and its surroundings is released into interstellar space. When enough stars explode near each other at the same time, the force of the explosion can push gas out of the galaxy itself in outflow winds that can travel at up to 2,000 kilometers per second.
“These galaxies are really interesting,” said Coyle, who is also chair of the Department of Astronomy and Astrophysics. “They occur when two large galaxies collide. The merger forces all the gas into a very small area, causing intense star formation. Large stars quickly burn out, and as they die, gas flows out. It is released as a wind.
Large quantity, rare, unknown origin
Technological developments have enabled ASKAP to scan large portions of the sky in very dark limits, allowing it to detect odd radio circles (ORCs) for the first time in 2019. The ORC is huge, hundreds of kiloparsecs across, and one kiloparsec is equivalent to 3,260 lights. year (for reference, milky way The galaxy is about 30 kiloparsecs in diameter).
Several theories have been proposed to explain the origin of ORCs, including planetary nebulae and nebulae. Black Hole However, radio data alone could not distinguish the theories. Intrigued, Coyle and her collaborators thought the radio rings could have originated from the later stages of the starburst galaxies they were studying. They began investigating her ORC 4. ORC is the first discovered ORC observable from the Northern Hemisphere.
Until then, ORCs had only been observed by radio emission, and no optical data were available. Coyle’s team observed ORC 4 using the Integrating Field Spectrometer at W. M. Keck Observatory on Mauna Kea, Hawaii, and found that it contained a huge amount of high-brightness, much more than is seen in the average galaxy. A heated compressed gas was revealed.
With more questions than answers, the team set to work doing some detective work. Using optical and infrared imaging data, they determined that the stars in the ORC 4 galaxy are about 6 billion years old. “This galaxy experienced an explosion of star formation that ended about a billion years ago,” Coyle said.
Simulation and conclusion
Cassandra Rojas, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics who specializes in theoretical aspects of galactic winds and co-author of the paper, created a series of numbers to recreate the size and properties of large radio waves. A computer simulation was performed. This ring contains a large amount of shocked cold gas in the central galaxy.
Her simulations showed that the outflowing galactic winds would continue to blow for 200 million years before stopping. When the winds stopped, forward shocks continued to push hot gas out of the galaxy, forming a radio ring, while reverse shocks caused cooler gas to fall into the galaxy. The simulation ran over her 750 million years. This is within ORC 4’s estimated stellar age of 1 billion years.
A computer simulation of a galactic wind outflow launched with an initial velocity of 450 kilometers per second and a mass outflow rate of 200 solar masses per year. This causes gas to be blown out of the galaxy into the surrounding paragalactic medium over a period of 200 million years. The left panel shows the gas temperature and the right panel shows the gas density. This simulation provides a possible explanation for the origin of odd radio circles.Credit: Cassandra Lochhaas / Space Telescope Science Institute
“For this to work, you need a high mass escape rate, which means you’re ejecting a lot of material very quickly. And the surrounding gas just outside the galaxy has to be low density. Otherwise, the shock stalls. Those are the two key factors,” said Coyle. “We found that the galaxies we have studied have high rates of mass outflow. Although rare, they do exist. This points to ORC originating from some type of outflow galactic wind. I really think so.”
Outflow winds not only help astronomers understand ORCs, but ORCs also help astronomers understand outflow winds. “ORC provides a way to ‘see’ the wind through radio data and spectroscopy,” Coyle said.
“This will help us determine how common extreme outflow winds are from galaxies, and what the wind lifecycle is like. These will help us learn more about the evolution of galaxies. Also helpful: Do all giant galaxies go through an ORC phase? Do spiral galaxies become elliptical when they stop forming stars? I think there’s a lot we can learn about and from ORC .”
Reference: “Ionized gas extends beyond 40 kpc in odd radio circle host galaxies” Alison L. Coyle, Selena Perrotta, David SN Rupke, Cassandra Rochas, Christy A. Tremonti, Alex Diamond Stanik, Drummond Fielding, James E. Geach, Ryan C. Hickox, John Moustakas, Gregory H. Rudnick, Paul Sell, Kelly E. Whalen, January 8, 2024. Nature.
DOI: 10.1038/s41586-023-06752-8