Astronomers discover ‘unexpected class’ of four mysterious circular objects in deep space, 3 have bright edges
The new class of astronomical object has been dubbed as odd radio circles or ORCs. These objects were discovered in the pilot survey of the Evolutionary Map of the Universe
Something strange seems to be lurking in the depths of space: astronomers have spotted never-seen-before circular objects deep in space and three of them are bright around the edges. Overall, four objects have been detected and all were found away from the Milky Way's galactic plane. According to the research team, they do not seem to correspond to any known type of object, but appear to be a “new class of astronomical object.”
“We found an unexpected class of astronomical object that have not previously been reported, consisting of a circular disc, which in some cases is limb brightened, and sometimes contains a galaxy at its center. The objects appear in radio images as circular edge-brightened discs,” says the research team in their pre-print, which can be accessed at arXiv. The paper has been submitted for publication to the journal Nature Astronomy, where it is currently under review. The study includes experts from Western Sydney University, Australia; CSIRO Astronomy & Space Science, Australia; National Radio Astronomy Observatory, US; Curtin University, Australia; and University of Wisconsin-Milwaukee, US, among others.
The objects have been dubbed “odd radio circles, or ORCs.” According to the researchers, circular features are well-known in radio astronomical images, and usually represent a spherical object such as a supernova remnant, a planetary nebula, a circumstellar shell, or a face-on disc such as a protoplanetary disc or a star-forming galaxy. “They may also arise from imaging artifacts around bright sources caused by calibration errors or inadequate deconvolution. Here we 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. For brevity, and lacking an explanation for their origins, we dub these objects “Odd Radio Circles,” or ORCs,” write authors.
These objects were discovered in the pilot survey of the Evolutionary Map of the Universe (EMU), which is an “all-sky continuum survey” using the newly-completed Australian Square Kilometre Array Pathfinder telescope (ASKAP). Three of the objects were found when researchers looked through images taken from the survey. Another, which looks slightly different, was found in archival data that was originally taken in March 2013. “Three ORCs (ORCs 1-3) were discovered by visual inspection of the images from the survey. Their rarity, together with their low surface brightness, makes it unlikely that they could have been discovered in previous radio surveys. We discovered a further ORC (ORC 4) in archival data taken with the Giant MetreWave Radio Telescope (GMRT) in March 2013. In most respects it is very similar to ORCs 1-3, but differs in having a central radio continuum source,” the findings state.
Describing their features, the team says the ORCs are strikingly circular. The objects are about one arcminute in diameter, for comparison, the moon's diameter is 31 arcminutes. “The four ORCs discussed here are similar in displaying a strong circular symmetry. They are also similar in having a diameter of about 1 arcmin, having a steep spectral index, being at high galactic latitude. They differ in that two of them have a central galaxy while two do not, and three of them (ORCs 1, 2 & 4) consist of a partly filled ring while one (ORC 3) seems to be a uniform disc. ORC 1, ORC 2, and ORC 4 are similar in having a filled but edge-brightened disc, while ORC 3 decreases monotonically from the center. There is also the puzzling fact that two of them are very close together, implying that these two ORCs have a common cause,” they write.
The astronomers explored several possible explanations, assuming all had a similar source, but none fit the bill. After ruling out supernova remnant, star-forming galaxy or ring galaxy, planetary nebulas, and cluster halo, among others, the astronomers speculate that the objects may represent a spherical shock wave from an extragalactic transient event, or the outflow, or a remnant, from a radio galaxy viewed end-on. According to scientists, more work is needed to investigate the source of these mystery objects.
“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,” say experts. They further write, “We also acknowledge the possibility that the ORCs may represent more than one phenomenon, and that they have been discovered simultaneously because they match the spatial frequency characteristics of the ASKAP observations, which occupy a part of the observational parameter space which has hitherto been poorly studied.”