Scientists have captured the best images yet of gigantic and unexplained rings in space known as Odd Radio Circles (ORCs), which stretch across astonishing distances of a million light-years and surround whole galaxies.
The bizarre glowing circles have perplexed astronomers since they were first discovered in 2020 by an ultra-sensitive radio telescope called the Australian Square Kilometre Array Pathfinder (ASKAP). A total of five ORCs have been spotted over the past two years, each spanning hundreds of thousands of light-years, suggesting that they might be fading remnants of extreme explosions generated by galaxies located at their centers, though the nature of such eruptions is not well understood.
To get a better read on the possible origins of ORCs, scientists led by Ray Norris, an astrophysicist at Western Sydney University and Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), followed up on the ASKAP observations with another extremely sensitive radio telescope called MeerKAT, which is located in South Africa.
Norris and his colleagues captured “detailed new MeerKAT radio images” of the first known ORC, called ORC J2103-6200 or ORC1, which [establish] its reality beyond doubt” and further clarifies “potential causes of the ORC phenomenon,” according to a study published on Tuesday in the Monthly Notices of the Royal Astronomical Society.
“We have imaged the first ORC with the MeerKAT telescope, resulting in a high resolution, high sensitivity image which shows internal structure that was not apparent in previous imaging,” the team said in the study. The researchers also captured new details imprinted in the light “which provides significant constraints on potential models of ORCs” while “also [studying] the properties and environment of the central host galaxy.”
The new observations with MeerKAT confirm that there’s a faint host galaxy at the center of ORC1, which is also a feature seen in the other known ORCs. The presence of these central hosts strongly hints that ORCs are some form of radio emission that gets blown into deep space by energetic galactic processes. The mechanism behind this eruption must be extremely powerful, as ORC1 is about 16 times wider than our own galaxy, the Milky Way, making it visible to telescopes even at a distance of more than a billion light years from Earth.
With the help of the new MeerKAT observations, Norris and his colleagues homed in on three possible explanations for ORCS: The first suggests that the rings are blown out by the tumultuous merger of two supermassive black holes in the host galaxy, the second posits that the structures are remnants of jets shot out by a supermassive black hole, also known as an active galactic nuclei (AGN), and the third proposes that ORCs are the afterglow of a recent “starburst” period of the host galaxy’s history that was defined by the sudden production of so many new stars that it created a shockwave.
The team found “strong circumstantial evidence” that “the host galaxy is probably an elliptical galaxy, with very little current star formation, but with a strong starburst within the last few [billion years],” according to the study. “The radio luminosity, together with the central compact source seen in the optical [wavelengths], then suggest that a radio-loud AGN lies at the centre of this galaxy.”
“The ORC lies in an overdensity of galaxies with several nearby companions that lie within the sphere of the ORC, and in future work we will explore whether these may be responsible for the arcs of radio continuum emission seen within the ORC,” the researchers added.
In other words, all three of these ORC origin stories are supported by some of the new observations with MeerKAT, yet “none of these models adequately explains the internal radio structure,” the team noted.
The new study brings us one step closer to unraveling the extraordinary mechanisms that create these enormous rings, plus it provides a stunning snapshot of ORC1. But the exact origins of these strange structures remain a mystery for the time being, so scientists will need to continue observing known ORCs and searching for more examples of these objects in the future.
These questions may be resolved by the world’s most sensitive radio telescope, known as the Square Kilometre Array (SKA), which encompasses both the MeerKAT and ASKAP arrays and is scheduled to be completed by the end of the 2020s.
“No doubt the SKA telescopes, once built, will find many more ORCs and be able to tell us more about the lifecycle of galaxies,” Norris said in a statement. “Until the SKA becomes operational, ASKAP and MeerKAT are set to revolutionize our understanding of the universe faster than ever before.”