Astronomers don’t usually say “What’s that?” when looking at the night sky. They know what most things are like stars, planets, black holes and galaxies. But in 2019, a new radio telescope called ASKAP saw something it had never seen before – huge circles made of radio waves, with whole galaxies inside them.

Scientists wanted to know what these circles were and what caused them. Now, a team led by Dr. Alison Coil thinks they have the answer. Dr. Coil studies galaxies with strong bursts of new stars forming. When massive stars die as supernovas, they blast gas from the star into space very fast. If enough stars explode close together, the force pushes gas out of the galaxy in galactic winds, which can travel over 2,000 km/second.

Dr. Coil’s team studies galaxies where two big galaxies crashed together, pushing gas into a small area. This triggers lots of new star formation. Massive stars burn fast and their explosions send out galactic winds.

Advances in technology let ASKAP see very faint radio signals. This revealed strange radio circles (ORCs) for the first time in 2019. ORCs were huge – hundreds of light years across. Many ideas tried to explain ORCs but the radio data alone couldn’t say which was right. Dr. Coil’s team thought the rings might be from the galaxies they studied.

They started looking at ORC 4, the first ORC seen from the northern sky. Before, only radio waves from ORCs were seen with no other data. Dr. Coil’s team used a special instrument at the Keck Observatory in Hawaii to observe ORC 4. It revealed a huge amount of hot, dense gas – far more than a normal galaxy holds.

The team dug deeper. Optical and infrared images showed stars in ORC 4’s galaxy were around 6 billion years old. There was strong star formation in this galaxy, but it ended about 1 billion years ago, said Dr. Coil.

Postdoctoral researcher Cassandra Lochhaas used simulations to recreate the giant radio ring’s size and properties over 750 million years, matching ORC 4’s star age. Her simulations showed galactic winds blew for 200 million years before stopping. Winds created shocked gas that formed the radio ring.

For this to work you need lots of gas leaving very fast, says Dr. Coil. The galaxies we’ve studied have just that. I think ORCs come from powerful galactic winds. ORCs may also help understand these extreme winds and how galaxies evolve over time. Dr. Coil thinks more can be learned by studying these rare, mysterious rings in the sky.


University of California – San Diego | Coil, A.L., Perrotta, S., Rupke, D.S.N. et al. Ionized gas extends over 40 kpc in an odd radio circle host galaxy. Nature (2024).

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