A strange shape has appeared in the Universe: a double luminous ring, invisible to the naked eye, but perfectly discernible to radio telescopes. This colossal structure, larger than several galaxies combined, could redefine our understanding of the extreme phenomena that stir the cosmos.
Discovered through an unprecedented alliance between professional researchers and amateur astronomers, this "Odd Radio Circle"—or ORC—represents the most powerful and distant example ever observed. Its light, captured today, has traveled for more than seven billion years before reaching Earth. This finding, published in the journal
Monthly Notices of the Royal Astronomical Society, sheds more light on the origin of these mysterious radio rings that first appeared in observations in 2019.
Figure showing the radio source RAD J131346.9+500320 observed at 144 MHz with an angular resolution of 6".
(a) LoTSS image, where only emission above the noise (50 μJy/beam) is visible; the white cross indicates the host galaxy.
(b) Radio map enhanced by horizontal and vertical Sobel filter, highlighting brightness gradients and filamentary structures.
(c) LoTSS radio contours superimposed on the BASS optical image; two green circles indicate the rings, and the dotted cyan circles mark galaxies of similar redshift.
(d) Same view as (c), but with galaxies color-coded according to their redshift; the two large dotted green circles again indicate the rings. A cosmic phenomenon still poorly understood
ORCs appear as gigantic circles of radio emission surrounding a galaxy, sometimes ten to twenty times larger than the Milky Way. Their radiation can only be detected in the radio band of the electromagnetic spectrum. These structures are formed of magnetized plasma moving at speeds close to that of light.
Initial hypotheses linked their formation to shocks from the merger of supermassive black holes or collisions between galaxies. These events, capable of releasing immense amounts of energy, could generate radio shock waves visible over millions, or even billions, of light-years.
The new study proposes a different explanation: these rings would originate from powerful streams of particles, called "galactic superwinds." These winds would be emitted by active spiral galaxies, where the combined energy of the central black hole and star formation ejects matter in the form of radio jets.
A discovery resulting from participatory science
The object named RAD J131346.9+500320 was identified through the RAD@home Astronomy Collaboratory platform, a citizen science program launched by the University of Mumbai. This project associates volunteers with the analysis of astronomical images collected by state-of-the-art instruments.
Participants collaborated with researchers to examine data from the European radio telescope LOFAR, composed of hundreds of thousands of antennas distributed across several countries. Operating at very low frequencies, LOFAR allows probing the sky as it appeared billions of years ago.
This new ORC, located at a spectral redshift of 0.94 (when the universe was half its current age), features two intersecting rings, an extremely rare characteristic observed only once before. Its radio emission power and distance make it a key piece for understanding the evolution of active galaxies.
Cosmic cousins just as astonishing
Alongside this discovery, researchers have uncovered two other radio giants. The first, RAD J122622.6+640622, spans nearly three million light-years. Its jet, abruptly deflected, forms a ring of one hundred thousand light-years at its tip.
The second, RAD J142004.0+621715, measures about 1.4 million light-years. It features a similar ring at the end of one of its jets, accompanied by a narrower counter-jet on the opposite side of the host galaxy.
These two objects reside in galaxy clusters containing hot plasma at millions of degrees. The interactions between the relativistic matter jets and this dense environment seem to shape these spectacular forms, linking their appearance to the dynamics of their surroundings.
Article author: Cédric DEPOND