Clusters of galaxies are the largest gravitationally bound objects in the Universe. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant 'radio haloes'. A radio halo at low frequencies associated with the merging cluster Abell 521 is reported. This halo has an extremely steep radio spectrum that makes it difficult to detect with observations at 1.4 GHz. The spectrum supports a turbulent acceleration mechanism, which predicts that many radio haloes in the Universe should emit mainly at low frequencies.
- G. Brunetti
- S. Giacintucci
- M. Markevitch
