The image shows 1,284 MHz emission from ORC 1 at 6 arcsecond resolution in a mint green colour, obtained by Ray Norris and colleagues (Mon. Not. R. Astron. Soc., in the press). It is the result of a ten-hour on-source observation, and the sensitivity of MeerKAT uncovers a complex internal structure consisting of knots and arcs within the outer circle. Overlaying the MeerKAT emission on an optical Dark Energy Survey image reveals an elliptical galaxy coincident with the radio source at the centre of the outer ring, while other galaxies correspond with the presence of knots. By consulting a photometric redshift survey, the authors ascertain that the central galaxy is at a distance of z ~ 0.55, implying that the ORC extends to a diameter of ~520 kpc. The survey reveals that a handful of the galaxies surrounding the central galaxy in the image also lie within the ORC, while others are at different redshifts. The central galaxy shows some potential signs of interaction with these neighbours. Fitting a spectral energy distribution to available optical, near-infrared and radio data suggests that the central galaxy contains an active galactic nucleus that went through a strong starburst episode several billion years ago.
Given that the outer ORC ring is most likely synchrotron emission, the research team propose three likely scenarios for the origin of the structure: a starburst termination shock, an explosion in the central galaxy possibly due to the merger of two supermassive black holes or an end-on radio lobe. There are strong and weak points of all three models, but ultimately further observations or a 3D simulation will be needed to discern between them. Critically, any convincing scenario will need to explain the internal filamentary radio structures: the overdensity of galaxies within the ORC may be key there.
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