What's known about Odd Radio Circle appearance? What would they look like if we could see them? Are they transparent/translucent in radio, or opaque?

Question

update: March 2022:

• Phis.org: Astronomers reveal best image yet of mysterious odd radio circles in space
• Science Alert: Weird Circular Objects Observed in Space May Finally Be Explained
• MNRAS: Mysterious odd radio circle near the large magellanic cloud – an intergalactic supernova remnant?

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The Conversation's ‘Dancing ghosts’: a new, deeper scan of the sky throws up surprises for astronomers says:

We are getting used to surprises as we scan the skies in the Evolutionary Map of the Universe (EMU) project, using CSIRO’s new Australian Square Kilometre Array Pathfinder (ASKAP), a radio telescope that probes deeper into the Universe than any other. When you boldly go where no telescope has gone before, you are likely to make new discoveries.

A deep search returns many surprises

The Dancing Ghosts were just one of several surprises found in our first deep search of the sky using ASKAP. This search, called the EMU Pilot Survey, is described in detail in a paper soon to appear in the Publications of the Astronomical Society of Australia.

The first big surprise from the EMU Pilot Survey was the discovery of mysterious Odd Radio Circles (ORCs), which seem to be giant rings of radio emission, nearly a million light years across, surrounding distant galaxies.

It includes the image below, which shows radio emission in "retro bluegreen" as a fuzzy blob floating in front of distant stars and galaxies.

Two things caught my eye:

1. I've seen this object in several original Star Trek episodes I'm sure!
2. Represented this way it's transparent. You can see stars and galaxies right through it.

Question: What's known about Odd Radio Circle appearance? What would they really look like if we could see them? Are they transparent/translucent in radio, or opaque?

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The first ‘Odd Radio Circle’. Radio data are green and the white and coloured data show the optical background from the Dark Energy Survey. image created by Jayanne English from data from EMU and the Dark Energy Survey (Creative Commons licence)

Answer 1

The current situation is summarized in The Conversation; Academic rigour, journalistic flair's March 21, 2022 ‘Odd radio circles’ that baffled astronomers are likely explosions from distant galaxies:

We can now see each ORC is centred on a galaxy too faint to be detected earlier. The circles are most likely enormous explosions of hot gas, about a million light years across, emanating from the central galaxy.

MeerKAT sees a small blob of radio emission in the centre of the ring, which is coincident with a distant galaxy. We are now fairly certain this galaxy generated the ORC.

We see these central galaxies in other ORCs too, all at vast distances from Earth. We now think that these rings surround distant galaxies about a billion light years away, which means the rings are enormous – around a million light years across.

From modelling the faint cloudy radio emission that MeerKAT detects within the rings, it seems the rings are the edges of a spherical shell surrounding the galaxy, like a blast wave from a giant explosion in the galaxy. They look like rings instead of orbs only because the sphere appears brighter at the edges where there is more material along the line of sight, much like a soap bubble.

MeerKAT has also mapped the polarisation of the radio waves, which tells us about the magnetic field in the ring. Our polarisation image shows a magnetic field running along the edge of the sphere.

This suggests that an explosion in the central galaxy caused a hot blast to collide with the tenuous gas outside the galaxy. The resulting shock wave then energised electrons in the gas, making them spiral around the magnetic field, generating radio waves.

One big surprise from the MeerKAT result is that within the ring we see several curved filaments of radio emission. We still don’t know what these are.

But we do know that the sphere is so huge that it has swallowed up other galaxies as it blasted out from the central galaxy. Perhaps the filaments are trails of gas ripped off the galaxies by the passing shock wave?

The big question, of course, is what caused the explosion. We are exploring two possibilities.

One is that they were caused by the merging of two supermassive black holes.

Another possibility is that the central galaxy went through a “starburst” event, in which millions of stars were suddenly born from the gas in the galaxy.

Both black hole mergers and starburst events are rare, which accounts for why ORCs are so rare (only five have so far been reported).

The puzzle of ORCs is not solved yet, and we still have much to learn about these mysterious rings in the sky. So far, we have only detected them with radio telescopes – we see nothing from the rings at optical, infrared, or X-ray wavelengths.

To find out more, we need a tool even more sensitive than MeerKAT and ASKAP. Fortunately, the global astronomical community is building just such an observatory – the Square Kilometre Array (SKA), an international effort with telescopes in South Africa and Australia.

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