The initial stages of cluster mergers observed in radio and X-rays

We present recent results based on radio and X-ray observations of two galaxy cluster pairs in a pre-merger phase: A1758N-A1758S (z=0.28) and RXCJ1825-CIZAJ1824 (z=0.07). In the former system, the deep LOFAR images reveal the presence of a giant bridge of radio emission connecting the two galaxy clusters on a scale of ~2 Mpc. This is the second large-scale radio bridge observed to date in a cluster pair. In contrast, no diffuse radio emission has been detected between RXCJ1825 and CIZAJ1824 possibly due to the lower energy dissipated by this pre-merging system (which is less massive than A1758N-A1758S). However, the new radio halo discovered in RXCJ1825 shows a low surface brightness extension likely generated during the interaction between the cluster and an infalling galaxy group (confirmed by optical and X-ray observations).
The reported results represent the first investigation of non-thermal phenomena in clusters in the early phase of collision. The detection of radio bridges and extensions of radio halos demonstrate the existence of magnetic fields at large distance from the cluster center, with potential implications on the evolution of galaxies in cluster outskirts.