In recent years the unprecedented sensitivities provided by SKA precursors/pathfinders have started revolutionising our view even of previously well-known objects such as jetted AGN. In particular, observations in the MHz-frequency regime are able to unveil the oldest plasma injected by AGN jets in their surrounding medium, providing new insights into the jet duty-cycle and feedback, as well as into the interaction between the AGN fossil plasma and the external thermal gas over timescales of hundreds of Myr. This is critical for understanding the thermal evolution of galaxy groups/clusters and, in turn, the evolution of galaxies in the universe over cosmic times. In particular, here I will discuss two spectacular galaxy groups, Nest200047 and NGC507, whose radio and X-ray observations have recently given us a uniquely detailed view of how the old AGN fossil plasma can get transported throughout the system and shredded into a complex array of filamentary structures.
AGN fossil plasma in galaxy groups