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Jennifer Ouellette - Mar 30, 2022 12:24 pm UTC

Astronomers have been puzzling over the nature and origin of rare, mysterious radio circles in space ever since the objects were first discovered in 2019. Now, the high-resolution MeerKAT radio telescope in South Africa has captured one such circle in much greater detail, offering some helpful clues about this rare phenomenon. The image and accompanying analysis appeared in a preprint on Arxiv, and the paper has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

The discovery arose from the Evolutionary Map of the Universe (EMU) project, which aims to take a census of radio sources in the sky. Several years ago, Ray Norris, an astronomer at Western Sydney University and CSIRO in Australia, predicted the EMU project would make unexpected discoveries. He dubbed them "WTFs." Norris admitted in a 2020 piece for The Conversation that he expected those discoveries would arise from machine learning analysis, given the vast amounts of data involved. "But these discoveries were made with good old-fashioned eyeballing," he wrote.

One pair of eyeballs belonged to Anna Kapinska, an astronomer at the National Radio Astronomy Observatory (NRAO). While browsing through new radio astronomy data collected by CSIRO's Australian Square Kilometer Array Pathfinder (ASKAP) telescope, Kapinska noticed several strange shapes that didn't seem to resemble any known type of object. Following Norris' nomenclature, she labeled them as possible WTFs. One of those, per Norris, "was a picture of a ghostly circle of radio emission, hanging out in space like a cosmic smoke ring."

Other team members soon found two more weird round blobs, which they dubbed odd radio circles (ORCs). A fourth ORC was identified in archival data from India’s Giant MetreWave Radio Telescope, and a fifth was discovered last year in new ASKAP data. There are several more objects that might also be ORCs. Based on this, the team estimates there could be as many as 1,000 ORCs in all.

While Norris et al. initially assumed the blobs were just imaging artifacts, data from other radio telescopes confirmed they were a new class of astronomical object. They don't show up in standard optical telescopes, or in infrared and X-ray telescopes—only in the radio spectrum. Astronomers suspect the radio emissions are due to clouds of electrons. But that wouldn't explain why ORCs don't show up in other wavelengths.

All of the confirmed ORCs thus far have a galaxy at the center, suggesting this might be a relevant factor in how they form. And they are enormous, measuring about a million light-years across, which is larger than our own Milky Way. "We know ORCs are rings of faint radio emissions surrounding a galaxy with a highly active black hole at its center, but we don't yet know what causes them, or why they are so rare," said Norris.

Astronomers have posited several possibilities for what the objects might be. Perhaps they are supernova remnants or Einstein rings. Alternatively, they might be caused by jets of electrons emitted from the vicinity of a supermassive black hole, although the ORCs are much more circular than the clouds typically seen resulting from this phenomenon in radio galaxies. The most speculative proposal posited that ORCs might actually be the "throats" of wormholes.

According to Norris, ASKAP is ideal for surveying large areas of the sky, while MeerKAT is designed to zoom in on any interesting objects, so the two telescope arrays are complementary. This latest radio image from MeerKAT shows several smaller rings inside the larger outer circle. MeerKAT also mapped out the polarization of the radio wave, revealing a magnetic field along the edge of the sphere. This is consistent with an explosion in the central galaxy.

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

So what caused the explosion that led to the formation of ORCs? The new data is sufficient to rule out all but three possibilities. The first is that ORCs are the result of a shockwave from the center of a galaxy, perhaps arising from the merging of two supermassive black holes. Alternatively, they could be the result of radio jets spewing particles from active galactic nuclei. Finally, ORCs may be shells caused by starburst events ("termination shock"), which would produce a spherical shock wave as hot gas blasted out from a galactic center.

Determining which of these hypotheses is correct will take more data. There is a large collection of radio telescopes, dubbed the Square Kilometer Array (SKA), scheduled to come online in 2028, and Norris is confident that SKA will find many more ORCs, hopefully revealing more details about the life cycle of galaxies. That additional observational data might also enable astronomers to identify the strange curved filaments of radio emissions lurking within the outer ring.

In the meantime, several astronomers are enjoying having a new cosmic puzzle to solve. “People often want to explain their observations and show that it aligns with our best knowledge," said co-author Jordan Collier of the Inter-University Institute for Data Intensive Astronomy. "To me, it’s much more exciting to discover something new, that defies our current understanding.”

DOI: Monthly Notices of the Royal Astronomical Society, 2022. 10.1093/mnras/stac701  (About DOIs).

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