Invisible radio signals from the cosmos have revealed previously unknown phenomena from prebiotic molecules in a starburst about 250 million light-years from Earth to the true rotation of Mercury. But the most famous occurred on August 6, 1967, when a squiggly stretch of high-speed recordings occupying less than a quarter-inch of astronomer Jocelyn Bell Burnell’s radio-telescope readouts revealed the first sign of something strange — an unknown cosmic mystery.
The minuscule signal appeared over and over again in the same part of the sky and she realized she was looking at a cosmic mystery –a repeating string of radio pulses spaced a bit more than a second apart that were unlike anything anyone had ever seen before. Bell-Burnell had detected the first evidence of a pulsar LGM-1 for Little Green Men. They thought the pulses could possibly be a beacon from an alien source.
Fast forward to today –mysterious circles of radio waves have left astronomers who are part of a pilot survey for a new project called the Evolutionary Map of the Universe (EMU) baffled with no idea how they formed, or even how big or far away they are. They don’t seem to match anything that has been seen before in the cosmos. The researchers dubbed them Odd Radio Circles, or ORCs.
“Odd Radio Circles”
Ray Norris at Western Sydney University, Australia, and his colleagues found the faint signals of these unexplained objects using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, which was completed in 2018. The discovery was made as part of a project aiming to take a census of all the radio sources in the sky.
The researchers, led by Norris, discovered four of the circles as part of a project aiming to take a census of all the radio sources in the sky –three were discovered using ASKAP and the fourth was found in archival data collected by the Giant MetreWave Radio Telescope in India, which helped the astronomers to confirm the objects as real, rather than an anomaly caused by issues with the ASKAP telescope or an error in the analyses of the data.
“Whenever we have new search capabilities that come online, we can expect to discover the unexpected,” says Jason Wright at Pennsylvania State University. The researchers have done “a really good job of running through all the possibilities of what they could be, or at least everything they can think of – including whether they’re not real,” says Wright.
Two Harbor a Galaxy
The strange ORCs are symmetrical and their edges are brighter than their interior. Two of them harbor a galaxy in the middle. The team has no idea how big or far away they are,and are unable to compare them with objects of a known distance. Two appear close together in the sky, but their physical distance from each other is unknown.
All four ORCs are bright at radio wavelengths but invisible in visible, infrared and X-ray light. But two of the ORCs have galaxies at their center that can be seen at visible wavelengths, which suggests that these objects might have been formed by those galaxies . Two ORCs also appear to be very close together, meaning their origins could be linked.
It isn’t clear if all four objects have a similar origin, and so far the researchers have no explanation for what they are, although they were able to confirm that the objects aren’t just artifacts of the ASKAP telescope by observing them through other instruments.
The team says that ORCs, found away from the Milky Way’s galactic plane and are around 1 arcminute across (for comparison, the moon’s diameter is 31 arcminutes), most closely resemble supernova remnants. But Mikako Matsuura at Cardiff University, UK, says that is unlikely.
“We know how many stars there are, due to past radio surveys,” says Matsuura about her observation that the that ORCs most closely resemble supernova remnants.The researchers note that there are only 350 confirmed supernova remnants in the galaxy and the likelihood of finding three of them in the ASKAP survey is 0.02 per cent.
Another possibility, says Kristine Spekkens, the Canadian science director for the Square Kilometre Array, is that the objects could be shockwaves leftover from some extragalactic event or possibly activity from a radio galaxy. Adding that all four ORCs are bright at radio wavelengths but invisible in visible, infrared and X-ray light. But two of the ORCs have galaxies at their center that can be seen at visible wavelengths, which suggests that these objects might have been formed by those galaxies . Two ORCs also appear to be very close together, meaning their origins could be linked.
Appear Only as Radio Waves
Another discounted explanation is that they are simply ring galaxies that are usually easy to observe with optical telescopes The ORCs however, only appear as radio waves.
“If these findings are as interesting as they seem to be, then people will use other telescope arrays to go looking for them,” says Penn State’s Wribnt, or look for more in archival data. “Once you have a lot of them, you can then start collecting clues.”
Image credit top of page: a composite image of the collisional ring galaxy R5519 and its environment compiled from Hubble and ZFOURGE images. Image credit: Tiantian Yuan / NASA / ESA / Hubble / ZFOURGE