Su et al. report the serendipitous discovery of a large, low-surface-brightness radio bubble in 944 MHz continuum data from the ASKAP Evolutionary Map of the Universe (EMU) survey. The structure, centred on the elliptical galaxy LEDA 217397 at a redshift of z=0.040, spans ∼8.4 arcmin, corresponding to a projected diameter of ∼399 kpc, and consists of two partly overlapping shells both with radii of ∼114 kpc. They compare the bubble with odd radio circles (ORCs) and large radio shells, and discuss three scenarios for its origin: a starburst-driven wind, a merger-driven shock, and AGN jet-inflated bubbles. The starburst wind is disfavoured on energetic grounds, and neither a halo-scale merger shock nor a spherical nuclear blast wave can explain the unusually regular, double-shell geometry; a bipolar nuclear outburst – a relic AGN jet episode, possibly triggered by a supermassive-black-hole merger – provides the most natural explanation, with later shocks possibly re-energising the plasma. The team conclude deeper broad-band radio, polarimetric, spectroscopic and X-ray observations are needed to confirm its nature.

The image above shows a contour overlay of the EMU image on the DESI Legacy Imaging Surveys optical image. The host galaxy LEDA 217397 lies at the centre of the whole structure. A background galaxy “A”, at a redshift of z~0.08 is marked. Two eROSITA soft X-ray sources X1 and X2 within the bubble are marked with green and purple circles, these are considered to be most plausibly background AGNs or X-ray binaries.