In October 2023, the MeerKAT radio telescope in South Africa detected a peculiar celestial object, a radio circle designated ORC J0356–4216. This phenomenon, characterized by its distinct circular shape visible only in radio wavelengths, is among the most recently identified and least understood cosmic features. An international research team, led by Sam Taziaux from Ruhr University Bochum in Germany, has published a detailed analysis of this discovery, providing new insights into its nature.
ORC J0356–4216 exhibits a symmetrical double-ring structure with a diameter of approximately two arcminutes. This translates to a physical size of about 2.18 million light-years, positioning it as one of the largest structures ever observed in the universe. The circle is located within the elliptical galaxy WISEA J035609.67–421603.5, at a redshift of 0.494. Observations indicate that the rings possess a polarization of 20% to 30%, with magnetic field orientations closely following the ring contours. The radio spectra of these rings are steep, with spectral indices of -1.18 and -1.12, and they lack significant internal substructures.
Researchers are exploring several hypotheses for the origin of this enigmatic radio circle. A leading theory suggests a connection to past activity from an active galactic nucleus (AGN), which involves a supermassive black hole at a galaxy's center accreting matter and emitting powerful jets and winds. The infrared colors of the host galaxy are consistent with an AGN within an elliptical galaxy, supporting this hypothesis. Another prominent theory posits that the circle could be the result of a large-scale shock wave, potentially generated by the interaction or merger of galaxy groups. Such shock waves have been observed in other cosmic contexts, accelerating particles to near light speeds and emitting the detected radio waves.
This discovery is part of a growing series of observations of "odd radio circles" (ORCs), primarily detected by the MeerKAT telescope. These peculiar structures, defined by their bright radio emission rings at their edges, are invisible in other wavelengths such as visible light, infrared, or X-rays. Their unique detectability in radio wavelengths is attributed to synchrotron radiation, produced by high-energy electrons interacting with magnetic fields. The ongoing study of ORCs aims to unravel their origins and their role in galaxy evolution. The findings of this particular analysis were made available on the arXiv platform on September 5, 2025.