‘Odd Radio Circles’ That Baffled Astronomers Are Likely Explosions From Distant Galaxies

In 2019, my colleagues and I discovered spooky glowing rings in the sky using CSIRO’s ASKAP radio telescope in Western Australia. The rings were unlike anything seen before, and we had no idea what they were.

We dubbed them odd radio circles, or ORCs. They continue to puzzle us, but new data from South Africa’s MeerKAT telescope are helping us solve the mystery.

We can now see each ORC is centered 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.

Our paper showing these results has been peer-reviewed and accepted for publication by Monthly Notices of the Royal Astronomical Society.

A Closer Look

We now have beautiful images of one of these rings taken with South Africa’s MeerKAT radio telescope, which shows the ORC in stunning detail. MeerKAT sees a small blob of radio emission in the center 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 modeling 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.

Energetic Electrons

MeerKAT has also mapped the polarization of the radio waves, which tells us about the magnetic field in the ring. Our polarization 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 energized electrons in the gas, making them spiral around the magnetic field, generating radio waves.

Lines around the edge of the ORC show the direction of the magnetic field. A circular magnetic field like this indicates it has been compressed by a shock wave from the central galaxy. Created by Larry Rudnick from MeerKAT data.

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?

Colliding Black Holes or the Birth of Millions of Stars?

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. Such a “merger event” releases an enormous amount of energy, enough to generate the ORC.

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. Such a starburst causes hot gas to blast out from the galaxy, causing a spherical shock wave.

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.

Getting a Better View

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.

ASKAP and MeerKAT were built to test the sites and technology for the SKA. Quite apart from their role as precursors for the SKA, both telescopes have been hugely successful in their own right, making major discoveries in their first years of operation.

Their success in discovering and studying ORCs therefore bodes well for the SKA.

The two telescopes are also beautifully complementary—ASKAP is superb at surveying large areas of sky and finding new objects, while MeerKAT is unrivalled for zooming in on those objects and studying them with higher sensitivity and resolution.

The SKA promises to surpass both. No doubt the SKA will find many more ORCs, and will also be able to probe them to find out what they are telling us about the lifecycle of galaxies.The Conversation

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Image Credit: Jayanne English, using data from MeerKAT and the Dark Energy Survey

Ray Norris
Ray Norris
Ray Norris is a British/Australian astronomer in the School of Science at Western Sydney University and with CSIRO Astronomy and Space Science. He researches how galaxies formed and evolved after the Big Bang, and the process of astronomical discovery with large data volumes. He also researches the astronomy of Australian Aboriginal people. Ray was educated at Cambridge University, UK, and moved to Australia in 1983 to join CSIRO, where he became Head of Astrophysics in 1994, and then Australia Telescope Deputy Director, and Director of the Australian Astronomy MNRF, before returning in 2005 to active research. He currently leads an international project—the Evolutionary Map of the Universe—to image the faintest radio galaxies in the universe, using the new ASKAP radio telescope recently completed in Western Australia. He also leads the WTF project which is exploring machine learning techniques to discover the unexpected. He frequently appears on radio and TV, and has published a novel, Graven Images.
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