Ray Norris of Western Sydney University and his collaborators have announced the radio telescopic discovery of four circular objects in the cosmos. Three of them exhibit shiny edges. In two a central galaxy is perceived, and one of them is a continuous radio source; but the presence there of those galaxies could be accidental. The second and third circles, which are very weak and the only one whose edge shines less than the center, are very close to each other; perhaps they are associated.
Until now they have not been able to fit these objects into some known type of phenomenon. The team has submitted the study to Nature Astronomy; meanwhile, he has published it in the arXiv prepublications repository. Norris and his collaborators assume that the circles may be spherical shock waves caused by some very powerful event or by ordinary galaxies.
Not that the circles are completely unknown in space. Remains of supernovae, protoplanetary disks, or star-forming galaxies can be seen as circles if the viewing angle is appropriate. But Norris and his collaborators have examined numerous phenomena known to create such conformations, and have found none consistent with the characteristics of the circles they have now observed, except perhaps for shock waves.
For example, they rule out that they are planetary nebulae or supernova remnants for probabilistic reasons; or that they are Einstein circles (the effect of a gravitational lens), due to their excessive size to be so clearly circular; or a star-creating galaxy (where that activity is often in the form of a ring) or an annular galaxy, since the radius brightness of the circles now discovered would only be compatible with the galaxy being optically visible; or a radius lobe associated with a jet of particles from an active galaxy, either from the side (the morphology is not the common one) or from the front (it would then appear, in effect, as a circle, but a suitable galaxy should be seen in its center ); and so with some other possibilities.
Therefore, the authors believe, this is not a new category of an already known phenomenon (the jet seen from the front), nor the remains of an old flow from a radio galaxy. As for the shock wave, the possibility that seems most likely to them, could have been caused by very violent transient phenomena (rapid radio explosions, gamma-ray explosions or neutron star mergers), which taking into account the angular size of the circles would have occurred long ago, or it could be the wind-associated shock wave from a star-forming galaxy (a possibility for now only theoretical).
It is also possible, conclude Norris and his collaborators, that these are different phenomena, now captured at once thanks to the characteristics of the discovering instrument, the Australian Pathfinder Square Kilometer Network (ASKAP), which allow it to study domains that have been poorly explored up to now. .
ASKAP became operational less than ten years ago; new radio sources are being sought with it. She found, specifically, three of the objects;. the fourth, that of the central radio source, was found by tracking a file of radio images from another instrument. But the objects are confirmed by subsequent observations with a different radio telescope or by digging up archived data. This is why the authors discard that the circles may be mere observational artifacts caused by errors. They have given a provisional collective name to the four circular findings: “rare radio circles,” with ORC as an acronym in English.
Reference: “Unexpected Circular Radio Objects at High Galactic Latitude”, by Ray P. Norris et al., In arXiv: 2006.14805 [astro-ph.GA].