We aim at an unbiased census of the radio halo population in galaxy clustersand test whether current low number counts of radio halos have arisen fromselection biases. We construct near-complete samples based on X-ray andSunyaev-Zel'dovich (SZ) effect cluster catalogues and search for diffuse,extended (Mpc-scale) emission near the cluster centers by analyzing data fromthe National Radio Astronomy Observatory Very Large Array Sky Survey. We removecompact sources using a matched filtering algorithm and model the diffuseemission using two independent methods. The relation between radio halo powerat 1.4 GHz and mass observables is modelled using a power law, allowing for a'dropout' population of clusters hosting no radio halo emission. An extensivesuite of simulations is used to check for biases in our methods. Our findingssuggest that the fraction of targets hosting radio halos may have to be revisedupwards for clusters selected using the SZ effect: while approximately 60 percent of the X-ray selected targets are found to contain no extended radioemission, in agreement with previous findings, the corresponding fraction inthe SZ selected samples is roughly 20 per cent. We propose a simple explanationfor this selection difference based on the distinct time evolution of the SZand X-ray observables during cluster mergers, and a bias towards relaxed,cool-core clusters in the X-ray selection.
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