Radio Halos in future surveys in the radio continuum
  Giant radio halos (RH) are Mpc-scale synchrotron sources detected in a
significant fraction of massive and merging galaxy clusters.Their statistical
properties can be used to discriminate among various models for their origin.
Theoretical predictions are important as new radio telescopes are about to
begin to survey the sky at low and high frequencies with unprecedented
sensitivity. We carry out Monte Carlo simulations to model the formation and
evolution of RH in a cosmological framework by assuming that RH are either
generated in turbulent merging clusters, or are purely hadronic sources
generated in more relaxed clusters, "off-state" halos. The models predict that
the luminosity function of RH at high radio luminosities is dominated by the
contribution of RH generated in turbulent clusters. The generation of these RH
becomes less efficient in less massive systems causing a flattening of the
luminosity function at lower luminosities. This flattening is compensated by
the contribution of "off-state" RH that dominate at lower luminosities. By
restricting to clusters at z<0.6, we show that the planned EMU+WODAN surveys at
1.4 GHz have the potential to detect up to ~200 RH, increasing their number by
one order of magnitude. A fraction of these sources will be "off-state" RH that
should be found at flux level < 10 mJy, presently accessible only to deep
pointed observations. We also explore the synergy between the Tier 1 LOFAR
survey at 150 MHz and the EMU+WODAN surveys at 1.4 GHz. We predict a larger
number of RH in the LOFAR survey due to the high LOFAR sensitivity, but also
due to the existence of RH with very steep spectrum that glow up preferentially
at lower frequencies. These RH are only predicted in the framework of turbulent
re-acceleration models and should not have counterparts in the EMU+WODAN
surveys, thus the combination of the two surveys will test theoretical models.
