Radio Halos and the importance of future observations at low frequency
  Radio observations discovered large scale non thermal sources in the central
Mpc regions of dynamically disturbed galaxy clusters (radio halos). The
morphological and spectral properties of these sources suggest that the
emitting electrons are accelerated by spatially distributed and gentle
mechanisms, providing some indirect evidence for turbulent acceleration in the
inter-galactic-medium (IGM). Radio and X-ray surveys allow to investigate the
statistics of radio halos and unveil a bimodal behaviour of the radio
properties of galaxy clusters: merging clusters host radio halos and trace the
well known radio--X correlation, while more relaxed clusters do not host radio
halos and populate a region well separated from that spanned by the above
correlation. This appears consistent with the hypothesis that relativistic
electrons can be reaccelerated by MHD turbulence generated during cluster
mergers. In the context of this model the population of radio halos consists of
a mixture of halos with different spectral properties, most of them with very
steep spectrum and visible only at low radio frequencies. For this reason the
future LOFAR surveys may provide a robust test to this theoretical hypothesis.
