Two intersecting radio shells: relics of galaxy merger shocks ?

Abstract: We present the discovery of two intersecting radio shells, likely radio relics, surrounding a compact galaxy group dominated by a massive elliptical galaxy. The shells present as partial, edge-brightened rings with diameters of about 240" (720 kpc) each and resemble a pair of odd radio circles. The central galaxy, WISEA J184105.19-654753.8, which shows signs of interactions, is radio bright, has a stellar mass of 3.1 x 10^11 Msun (for a redshift of zphot = 0.18) and is located in the intersect region. The double radio shell system, which we refer to as ORC J1841-6547 (also known as ORC 6), was detected in 944 MHz radio continuum images obtained with Phased Array Feeds on the Australian Square Kilometre Array Pathfinder (ASKAP). The more prominent, north-western shell may be associated with an X-ray detection, while the weaker, south-eastern shell has no counterpart at non-radio wavelength. We propose outwards moving shocks from galaxy mergers driving into the intragroup medium, re-energising relic radio lobes, as a possible formation scenario for the observed radio shells. We conclude that at least some ORCs are shock-energised relics in the outskirts of galaxy groups, which originate during the merger evolution of the brightest group galaxy.