A low-frequency radio halo associated with a cluster of galaxies

Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10(15) solar masses of hot (10(8) K) gas, galaxies and dark matter in a typical volume of 10 Mpc(3). Magnetic fields and relativistic particles are mixed with the gas as revealed by giant 'radio haloes', which arise from diffuse, megaparsec- scale synchrotron radiation at cluster centre(1,2). Radio haloes require that the emitting electrons are accelerated in situ ( by turbulence)(3-6), or are injected ( as secondary particles) by proton collisions into the intergalactic medium(7-10). They are found only in a fraction of massive clusters that have complex dynamics(11-14), which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut- off; this makes the halo difficult to detect with observations at 1.4 GHz ( the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.