Galaxy clusters enveloped by vast volumes of relativistic electrons

The central regions of galaxy clusters are permeated by magnetic fields and filled with relativistic electrons1. When clusters merge, the magnetic fields are amplified and relativistic electrons are re-accelerated by turbulence in the intracluster medium2,3. These electrons reach energies of 1–10 GeV and, in the presence of magnetic fields, produce diffuse radio halos4 that typically cover an area of around 1 Mpc2. Here we report observations of four clusters whose radio halos are embedded in much more extended, diffuse radio emission, filling a volume 30 times larger than that of radio halos. The emissivity in these larger features is about 20 times lower than the emissivity in radio halos. We conclude that relativistic electrons and magnetic fields extend far beyond radio halos, and that the physical conditions in the outer regions of the clusters are quite different from those in the radio halos.