Circular structures are so common in astronomy that you might think there couldn't ever be anything odd about them. From galaxies and accretion discs (even planets if you count spheres), to supernovae remnants and stellar shells, being circular isn't especially strange in itself. Of course the problem is context. Galaxy collisions can produce some spectacular shells and truly circular rings if conditions are right, but generally they result in a big ugly mess. You don't, for instance, expect to find a neat ring lying right next to a galaxy that looks practically undisturbed, which is why Keenan's Ring is so weird.
Three of the ORCs reported here are found in a pilot survey for one of the big Square Kilometre Array precursor telescopes, ASKAP, and one from archival data of the GMRT. They're radio continuum sources, meaning they emit over a wide range of frequencies (800-1100 MHz). This, plus the fact that they don't have optical counterparts - at least nothing obvious - this makes it fiendishly difficult to estimate their distance. Three of them look like filled circular structures while one is a ring. Two of them are directly adjacent to each other while the rest are isolated. All are well-resolved by the telescopes. So these are particularly challenging things to study : very nice, clear images of strong detections, but almost devoid of any context at all that could help explain the little blighters.
Far from being absolutely total mysteries which break science, there are no less than eleven possible explanations listed here. The problem is that none of them are without issue. Since two of them are a pair, some common origin seems likely, but it's entirely possible that they're formed by a mix of different mechanisms. Likewise, two of them have possible optical counterparts, but nothing even close to secure. It's all very frustrating.
"Imaging artifact" can probably be ruled out since some were detected with different telescopes. Collisional ring galaxies also seem unlikely because there's no nice bright optical counterpart, nor do they match the typical characteristics of any of the various radio emission structures produced by AGN (active galactic nuclei) or the halo gas found in galaxy clusters.
There's a few possibilities left : supernovae remnants, planetary nebulae, or galactic winds. The first two they dismiss by the same argument : that the density of such features already known is so low that they wouldn't expect to detect any by chance in their survey. In my opinion this is the weakest part of the paper. Sometimes a weird discovery demands a weird explanation, so more discussion about the physical characteristics would have helped a lot here - otherwise it becomes very much a Bayesian vampire. And there should be some discussion on where such features are typically found; the surveys might be biased towards finding more than is typical.
The third, which they stop short of saying, "this is definitely the answer, we'll take the Nobel Prize now, thanks" but just barely is a galactic wind termination shock. The collective stellar wind from an actively star-forming galaxy could, they say, give rise to just the sort of features they see here, provided the galaxies are in homogeneous media. Okay, but why isn't the star forming galaxy clearly visible, hmm ? They conclude, "such a shock has not yet been observed elsewhere", which feels just a little bit like, "we like this one because it's cool".
I'm being somewhat facetious, of course. It's a good and very interesting piece of work and I hope we get more ghastly ORCs in the future. I'm not entirely sold on these things not having some boring mundane explanation, but I hope they don't.
Unexpected Circular Radio Objects at High Galactic Latitude
We have found an unexpected class of astronomical objects which have not previously been reported, in the Evolutionary Map of the Universe Pilot survey, using the Australian Square Kilometre Array Pathfinder telescope. The objects appear in radio images as circular edge-brightened discs about one arcmin diameter, and do not seem to correspond to any known type of object.
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