Diffuse radio emission at low frequencies in merging galaxy clusters

Extended diffuse radio features so-called radio halos radio relics and radio mini-halos are found in about 100 galaxy clusters. The radio emission originates from relativistic electrons suffering synchrotron losses in the magnetized intra-cluster medium. Little is known about the mechanisms how electrons in the intra-cluster medium get accelerated to relativistic energies. Observations at low radio frequencies permit the analysis of the distribution of the population of aged relativistic electrons. In this work we analyzed in the context of the WSRT 346 MHz Legacy Survey ten galaxy clusters with already known diffuse radio sources and yet unclassified sources. We confirm radio relics in three clusters radio halos in five clusters and classified a radio relic candidate as a radio galaxy. Furthermore we revealed radio features with remarkable morphology in the galaxy cluster Abell 2069. Abell 2069 is a complex merging galaxy cluster which shows two distinct X-ray peaks. The main cluster component evidently possesses a radio halo and the subcomponent exhibits ultra-steep spectrum diffuse radio emission of uncertain nature. We conducted follow-up observations on Abell 2069 with LOFAR in the frequency range from 120 to 180 MHz. We complemented our investigation of Abell 2069 with GMRT observations at 322 MHz and 608 MHz as well as with archival Chandra X-ray data. Our analysis confirms the presence of a radio halo in the main cluster component. Furthermore we classify the diffuse radio emission in the subcluster as radio mini-halo. We detected signatures of fossil plasma ejected during a former active phase of an Active Galactic Nuclei in the subcluster. A cold front in the subcomponent suggests the presence of internal gas sloshing which could have induced bulk motions and turbulence in the subcluster. Both mechanisms together may have redistributed and re-accelerated the fossil plasma and caused the diffuse radio emission.