New Class of Radio-Astronomical Objects Discovered: Odd Radio Circles (Astronomy)

An international team of astronomers has discovered an unexpected new class of radio-astronomical objects, consisting of a circular disk, which in some cases is limb-brightened, and sometimes contains a galaxy at its center. Named ‘Odd Radio Circles,’ these objects do not seem to correspond to any known type of astronomical object.

Fig: ASKAP radio continuum image of ORC 1 (contours) overlaid onto a DES 3-color composite image. Two galaxies of interest: ‘C’ lies near the center of ORC 1 and ‘S’ coincides with the southern radio peak. Image credit: Norris et al, arXiv: 2006.14805.

Circular features are well-known in radio-astronomical images, and usually represent a spherical object such as a supernova remnant, a planetary nebula, a shell around a star, or a face-on disk such as a protoplanetary disk or a star-forming galaxy.

They may also arise from imaging artifacts around bright astronomical objects.

Western Sydney University and CSIRO astronomer Ray Norris and his colleagues report the discovery of a class of circular feature in radio images that do not seem to correspond to any of these known types of object or artifact, but rather appear to be a new class of astronomical object.

The researchers spotted three ORCs — named ORCs 1, 2 and 3 — in images from the Pilot Survey of the Evolutionary Map of the Universe, which is an all-sky continuum survey using the Australian Square Kilometre Array Pathfinder telescope (ASKAP).

A further radio source, called ORC 4, was discovered in archival observations of the galaxy cluster Abell 2142 taken with the Giant MetreWave Radio Telescope (GMRT).

All four ORCs are similar in displaying a strong circular symmetry and none of them have obvious counterparts in optical, infrared and X-ray wavelengths.

They differ in that two of them have a central galaxy while two do not, and three of them (ORCs 1, 2 and 4) consist of a partly filled ring while one (ORC 3) seems to be a uniform disk. There is also the puzzling fact that two of them are very close together, implying that these two ORCs have a common cause.

If the central galaxy in ORC 4 is associated with the ring, then the ring is 4.2 billion light-years away and has a size of 1.1 by 0.9 million light-years.

Fig: ASKAP radio continuum images of ORCs 2 and 3 from the Pilot Survey of the Evolutionary Map of the Universe and of ORC 4 from GMRT archival data. On the left are gray-scale images, with the synthesized beam shown in the bottom left corner, and radio contours overlaid onto DES optical images on the right. Image credit: Norris et al.

They speculated that they may represent a spherical shock wave from an extra-galactic transient event. Several such classes of transient events, capable of producing a spherical shock wave, have recently been discovered, such as fast radio bursts, gamma-ray bursts, and neutron star mergers. However, because of the large angular size of the ORCs, any such transients would have taken place in the distant past.

It is also possible that the ORCs represent a new category of a known phenomenon, such as the jets of a radio galaxy or blazar when seen end-on, down the ‘barrel’ of the jet.

Alternatively, they may represent some remnant of a previous outflow from a radio galaxy.

However, no existing observations of this phenomenon closely resemble the ORCs in features such as the edge-brightening or the absence of a visual blazar or radio galaxy at the center.

References: Ray P. Norris, Huib T. Intema, Anna D. Kapinska, Baerbel S. Koribalski, Emil Lenc, L. Rudnick, Rami Alsaberi, Craig Anderson, G. E. Anderson, E. Crawford, Roland Crocker, Stefan W. Duchesne, Miroslav D. Filipovic, Andrew M. Hopkins, Natasha Hurley-Walker, Susumu Inoue, Kieran Luken, Peter Macgregor, Pero Manojlovic, Josh Marvil, Andrew N. OBrien, Wasim Raja, Devika Shobhana, Jordan D. Collier, Catherine Hale, Aidan Hotan, David McConnell, Vanessa Moss, Matthew Whiting, “Unexpected Circular Radio Objects at High Galactic Latitude”, pp. 1-32, 2020..

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