(ORDO NEWS) — In 2019, my colleagues and I discovered eerie glowing rings in the sky using CSIRO’s ASKAP radio telescope in Western Australia. The rings were unlike anything we had seen before, and we had no idea what they were.
We called them strange radio circles, or ORCs. They are still a mystery, but new data from South Africa’s MeerKAT telescope is helping us unravel the mystery.
We now see that at the center of each ORC is a galaxy too faint to have been detected before. Most likely, these circles are huge explosions of hot gas, about a million light-years across, emanating from the central galaxy.
Our paper with these results has been peer-reviewed and accepted for publication in the Monthly Notices of the Royal Astronomical Society.
We now have beautiful images of one of these rings taken by South Africa’s MeerKAT radio telescope, showing the ORC in stunning detail.
For example, MeerKAT sees a small bunch of radio emission at the center of the ring, which coincides with a distant galaxy. Now we are absolutely sure that this galaxy gave birth to the ORC.
We see these central galaxies in other ORCs as well, all at great distances from Earth. We now believe that these rings surround distant galaxies at a distance of about a billion light years, which means that the rings are huge – about a million light years across.
Simulations of the faint radio emission MeerKAT detects inside the rings suggest that the rings are the edges of a spherical shell surrounding the galaxy, like the blast wave from a giant explosion in the galaxy.
They only look like rings rather than balls because the sphere appears brighter at the edges, where there is more material along the line of sight, like a soap bubble.
Above: Artistic portrait of strange radio circles emerging from the central galaxy. The rings are thought to have taken 1 billion years to reach the size we see today. The rings are so large (millions of light-years across) that they have expanded beyond other galaxies. (Sam Moorfield/CSIRO)
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 along the edge of the sphere.
This suggests that the explosion in the central galaxy led to the collision of a hot shock wave with dense gas outside the galaxy. The resulting shock wave charged the electrons in the gas, causing them to spiral around the magnetic field, generating radio waves.
Top: Lines along the edge of the ORC show the direction of the magnetic field. Such a circular magnetic field indicates that it was compressed by a shock wave from the central galaxy.
The big surprise from MeerKAT is that inside the ring we see several curved filaments of radio emission. We still don’t know what it is.
But we do know that the sphere is so huge that as it erupted from the central galaxy, it swallowed up other galaxies. Perhaps the filaments are traces of gas torn off from galaxies by a passing shock wave?
Collision of black holes or birth of millions of stars?
The main question, of course, is what caused the explosion. We are exploring two possibilities.
One is that they were caused by the merger of two supermassive black holes. With such a “merger” a huge amount of energy is released, sufficient for the occurrence of ARC.
Another possibility is that there was a “stellar explosion” in the central galaxy, in which millions of stars were suddenly born from the gas in the galaxy. As a result of such a stellar explosion, hot gas erupts from the galaxy, causing a spherical shock wave.
Both black hole mergers and bursts of star formation are rare, which explains why ORCs are so rare (only five cases have been recorded so far).
The ORC mystery has not yet been solved, and we still have a lot to learn about these mysterious rings in the sky. So far, we have only detected them with radio telescopes – we do not see anything from the rings in the optical, infrared or X-ray wavelengths.
To learn more, we need an instrument even more sensitive than MeerKAT and ASKAP. Fortunately, the world astronomical community is building just such an observatory – the Square Kilometer Array (SKA), an international project with telescopes in South Africa and Australia.
ASKAP and MeerKAT were built to test sites and technologies for SKA. In addition to their role as predecessors to the SKA, both telescopes were extremely successful in their own right, making major discoveries in their early years.
Therefore, their success in discovering and studying ORCs bodes well for the SKA.
The two telescopes also complement each other perfectly: ASKAP excels at viewing large areas of the sky and finding new objects, while MeerKAT excels at zooming in on these objects and studying them with higher sensitivity and resolution.
SKA promises to outperform both systems. Undoubtedly, the SKA will discover many more ORCs, and will also be able to study them to find out what they tell us about the life cycle of galaxies. The Conversation
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