Galactic winds from exploding stars could explain the giant rings. Every day astronomers ask “What is this?” They don’t ask. After all, most of the observed astronomical phenomena are known: stars, planets, black holes, galaxies. But in 2019, the recently completed ASKAP (Australian Square Kilometer Array Pathfinder) telescope revealed something no one had ever seen before: rings of radio waves at their centers large enough to contain entire galaxies.

While the astrophysics community was trying to determine what these circles were, they also wanted to know: From where Now, a team led by UC San Diego astronomy and astrophysics professor Alison Coyle believes they may have found the answer: The rings are envelopes formed from galactic outflows, perhaps massive exploding stars known as supernovae. His work has been published at: Nature.

Coyle and his colleagues studied massive galaxies with starbursts that could drive these ultrafast outflow winds. Starburst galaxies have exceptionally high star formation rates. When stars die and explode, they spew gas from the star and its surroundings back into interstellar space. If enough stars explode near each other at the same time, the force of these explosions can push gas out of the galaxy into outflow winds that can travel at speeds of up to 2,000 kilometers per second.

“These galaxies are really interesting,” said Coyle, who is also chair of the astronomy and astrophysics department. “They occur when two massive galaxies collide. The merger pushes all the gas into a very small area, causing an intense burst of star formation. Massive stars burn rapidly and expel their gas as wind when they die.”

Massive, rare and of unknown origin

Technological advances have allowed ASKAP to scan large parts of the sky with very faint margins, leading to the discovery of strange radio circles (ORCs) for the first time in 2019. ORCs were huge; it was hundreds of kiloparsecs in diameter; where one kiloparsec was equal to 3,260 light-years (for reference, the Milky Way galaxy is about 30 kiloparsecs in diameter).

Various theories have been proposed to explain the origin of ORCs, including planetary nebulae and black hole mergers, but radio data alone could not distinguish between the theories. Coyle and his colleagues were intrigued and thought the radio rings might be an evolution of the later stages of the starburst galaxies they were studying. They began exploring ORC 4, the first open ORC observable from the Northern Hemisphere.

Until then, ORCs were observed only through radio emissions, without any optical data. Coyle’s team looked at ORC 4 using the Integral Field Spectrograph at the WM Keck Observatory in Maunakea, Hawaii; This revealed a huge amount of extremely bright, heated, compressed gas, far more than seen in the average galaxy.

With more questions than answers, the team began their detective work. Using optical and infrared imaging data, they determined that the stars in the ORC 4 galaxy are approximately 6 billion years old. “There was a burst of star formation in this galaxy, but that ended about a billion years ago,” Coyle said.

Modeling and results

Cassandra Lohaas, a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics who specializes in the theoretical side of galactic winds and co-author of the paper, ran a series of numerical computer simulations to reconstruct the size and properties of the large-scale radio. The ring also contains large amounts of shocked cold gas in the central galaxy.

Their simulations showed that the original galactic winds blew for 200 million years before shutting down. When the wind stopped, the forward shock continued to push hot gas out of the galaxy and form a radio ring, while the reverse shock sent cooler gas back into the galaxy. The simulation took place over a period of more than 750 million years, within one billion years, the estimated stellar age of ORC 4.

“For this to work, you need a high flow rate, which means a lot of material is being expelled very quickly. And the surrounding gas outside the galaxy has to be of low density, otherwise the shock would stop. Those are two important factors,” Coyle said. “It turns out that the galaxies we studied have very high rates of mass flux. They’re rare, but they do exist. I think this really indicates that the ORCs are coming from some kind of galactic wind.”

Not only can source winds help astronomers understand ORCs, but ORCs can also help astronomers understand source winds. “ORCs give us the ability to ‘see’ winds with radio data and spectroscopy,” Coyle said.

“This can help us determine how common extreme progalactic winds are and what the life cycle of the wind is. They can also help us learn more about galaxy evolution: Do all large galaxies go through an ORC phase? Do spiral galaxies become elliptical when they no longer form stars?” ?I think we can learn a lot about ORC and learn from ORC.”