Top: Number of sources per redshift. The bin size is 0.1. Middle: Radio luminosity at 1.4 GHz versus redshift. The redshift plotted is that of the galaxy groups. The radio luminosity is calculated from the 1.4 GHz flux density for the redshift of the object. Black represents all group galaxies, red is for BGGs, and yellow is for SGs (see Sect. 2 for clarification on the classification). Bottom: Halo mass versus redshift. Pink filled squares denote log10(M₂₀₀/M_⊙) > 13.5 and yellow open circles log10(M₂₀₀/M_⊙) < 13.5, which we refer to as log10(M₂₀₀/M_⊙)≈13.3 in the rest of the paper. The divide shows our adopted halo mass cut to account for sample completeness (Sect. 2.2).