Gentle re-energisation of electrons in merging galaxy clusters

Galaxy clusters are the most massive constituents of the large-scale structure of the Universe. While the hot thermal gas that pervades galaxy clusters is relatively well understood through observations with X-ray satellites, our understanding of the non-thermal part of the intra-cluster medium remains incomplete. With LOFAR and GMRT observations, we have identified a phenomenon that can be unveiled only at extremely low radio-frequencies and offers new insights into the non-thermal component. We propose that the interplay between radio-emitting plasma and the perturbed intra-cluster medium can gently re-energise relativistic particles initially injected by active galactic nuclei. Sources powered through this mechanism can maintain electrons at higher energies than radiative ageing would allow. If this mechanism is common for aged plasma, a population of mildly relativistic electrons can be accumulated inside galaxy clusters providing the seed population for merger-induced re-acceleration mechanisms on larger scales such as turbulence and shock waves.