We have our best look yet at mysterious ORCs (odd radio circles) in space

Astronomers have been puzzling over the nature and origin of rare, mysterious radio circles in space since the objects were first discovered in 2019. Now the high-resolution MeerKAT radio telescope in South Africa has captured such a circle in much more detail and offers some useful 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 came out of the Evolutionary Map of the Universe (EMU) project, which aims to keep a count of radio resources in the air. Several years ago, Ray Norris, an astronomer at Western Sydney University and CSIRO in Australia, predicted that the EMU project would make unexpected discoveries. He called them “WTFs.” Norris admitted in a 2020 piece for The Conversation that he expected those discoveries to come from machine learning analysis, given the vast amounts of data involved. “But these discoveries have been made with a good old-fashioned look,” he wrote.

One pair of eyeballs belonged to Anna Kapinska, an astronomer at the National Radio Astronomy Observatory (NRAO). While browsing new radio astronomy data collected by CSIRO’s Australian Square Kilometer Array Pathfinder (ASKAP) telescope, Kapinska noticed several strange shapes that did not resemble any known type of object. Following Norris’ nomenclature, she labeled them as possible WTFs. One, according to Norris, “was a photograph of a ghostly circle of radio emissions, which hung in space like a cosmic ring of smoke.”

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

while Norris et al. initially assumed that the blobs were just image artifacts, data from other radio telescopes confirmed that they were actually a new class of astronomical objects. They don’t appear 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 at other wavelengths.

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

Astronomers have posited several possibilities for what the objects might be. Maybe they’re supernova remnants or Einstein rings. Alternatively, they could be caused by electron beams emitted from the vicinity of a supermassive black hole, although the ORCs are much more circular than the clouds commonly seen as a result of this phenomenon in radio galaxies. The most speculative proposal suggested that ORCs could actually be the “throats” of wormholes.

According to Norris, ASKAP is ideal for examining large areas of the sky, while MeerKAT is designed to zoom in on objects of interest, so the two telescope arrays are complementary. This latest radio image from MeerKAT shows several smaller rings within the larger outer circle. MeerKAT also mapped the polarization of the radio wave and revealed a magnetic field along the rim of the sphere. This corresponds to an explosion in the central galaxy.

“We can now see that each ORC is centered on a galaxy too faint to be detected earlier,” Norris wrote in a new article in The Conversation. “The circles are most likely huge explosions of hot gas, about a million light-years across, emanating from the central galaxy. Modeling the faint cloudy radio emission that MeerKAT detects within the rings, it appears that the rings are the fringes. of a spherical shell surrounding the galaxy, like a blast wave from a giant explosion in the galaxy.They look like rings instead of spheres, just because the sphere appears brighter at the edges where there is more material along the line of sight, just like a bubble.”

So what caused the explosion that led to the formation of ORCs? The new data is enough to rule out all but three possibilities. The first is that ORCs are the result of a shock wave from the center of a galaxy, possibly created by the merging of two supermassive black holes. Alternatively, they could be the result of radio beams spewing particles from active galactic nuclei. Finally, ORCs could be grenades triggered by starburst events (“termination shock”), which would produce a spherical shock wave when hot gas is shot from a galactic center.

More data is needed to determine which of these hypotheses is correct. There is a large collection of radio telescopes called the Square Kilometer Array (SKA) that is expected 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 may also enable astronomers to identify the strange curved filaments of radio emissions lurking in the outer ring.

Meanwhile, several astronomers enjoy solving a new cosmic puzzle. “People often want to explain their observations and show that it matches our best knowledge,” said study co-author Jordan Collier of the Inter-University Institute for Data Intensive Astronomy. “For me, it’s much more exciting to discover something new, which defies our current understanding.”

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