Astronomers Can't Explain The Cause Of “Odd Radio Circles”

An enhanced image of the original ORC reveals its ghostly structure, including the apparent non-radiating areas. Norris et al/Arxiv.org

Newly discovered circular areas of sky that emit radio waves without any obvious source are puzzling astronomers, with every explanation anyone can think of ruled out. Since the first discovery last year, enough of these objects have been found to convince astronomers they are a genuine phenomenon rather than an aberration or equipment error. However, we know little else about them, other than that they are almost certainly located far outside our own galaxy and are absolutely enormous.

The quest to study the skies with new equipment has been turning up hard-to-explain findings ever since Galileo called Saturn's ring the planet's “ears”. When Jocelyn Bell Burnell detected pulsars in the early days of radio astronomy, they were briefly labeled LGMs for “Little Green Men”.

In this fine tradition, astronomers at the Australian Square Kilometer Array Pathfinder Telescope (ASKAP) initially referred to strange circular patches of radio waves as “WTF”, which one finder unconvincingly explained as “Widefield ouTlier Finder”. They've now moved to the slightly more formal “Odd Radio Circles” (ORCs) for the four confirmed discoveries, with several more suspected.

ORCs are odd partly because they've only been seen in radio waves, although some of them surround distant galaxies visible at other wavelengths. Another strange feature is their near-perfect shape. “We don't see circles like this any other way, except supernova remnants," Professor Ray Norris of Western Sydney University told IFLScience. “And these are no supernova remnants.”

That much was suspected from the beginning since the ORCs lie a long way from the plane of the galaxy where most nearby exploding stars lie. The galaxies at the centers of some of the circles are around a billion light-years away, and Norris told IFLScience that “one might have been a coincidence, but we've now seen enough to be confident the galaxies are the source.” That, however, makes the ORCs enormous, around a million light-years across, 10 times the diameter of our own galaxy, Norris reports in a paper accepted for Nature Astronomy (preprint on ArXiv.org). We have no idea what could create something so large.

One of the more puzzling features of ORCs is that out of only four confirmed, ORC2 and ORC3 are next to each other. Norris et al/Arxiv.org

This size rules out most of the obvious explanations. Norris admits it's possible we are seeing something familiar like a radio galaxy's jets from an odd angle but thinks it far more likely the cause is something completely new.

Examining the ORCs at different wavelengths has provided some clues. After originally being found at wavelengths of about 30 centimeters (12 inches), the ORCs have been detected from 1 meter to 15 centimeters (39 to 6 inches). The telescopes used so far can't see wavelengths shorter than that and lose resolution for longer wavelengths, so time will need to be observed on other equipment to extend the range. The pattern of stronger signals at longer wavelengths seen so far is consistent with electrons cooling over time, Norris added, but what could spray hot electrons over such a vast area remains a mystery.

“I've waited my whole life to find something like this,” Norris told the BBC. “I wish I was like 20 or something and had the rest of my life to study them.”

The ORCs have only just been discovered because their faintness and angular size required radio telescopes that are both large and can map immense areas of the sky. The arrival of the ASKAP made the discoveries possible, but they have since been confirmed using other instruments so we know this is not a glitch inherent in this machine.

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