We've mapped the entire sky in multiple wavelengths of light and in many cases to decently high resolution, and yet there is still stuff out there popping up in images that's just downright bizarre, things which aren't easily explained.
Like, what the heck are these weird faint circular structures turning up in deep radio surveys?
Radio images (left column) of ORCs 1 (top), 2 and 3 (center), and 4 (bottom), with contours of radio strength (right; like a contour map of topography where the lines are closer together the radio emission is stronger). Credit: Norris et al.
The astronomers who found them call them ORCs, for Odd Radio Circles — I have to give them credit for that, it cuts to the chase — and skipping ahead a chapter or two: They don't know what they are.
They were found initially in a pilot survey for a project called the Evolutionary Map of the Universe, or EMU, being done on the Australian Square-Kilometre Array Pathfinder telescope (so the name EMU makes sense, then; I like this team). They're observing a 270 square degree patch of sky (roughly half the size of your fist held at arm's length) in radio waves ...