Redshift z versus cluster-centric distance for the catalogue galaxies with z <  0.06 (filled squares). The blue solid lines represent the cluster membership criterion adopted in the present study. The dashed horizontal line marks the median redshift of the selected cluster members (blue filled squares). Green open squares indicate galaxies classified as Hα emitters from Hα imaging surveys (Sect. 4.3). At the distance of the Perseus cluster, 140′ correspond to ∼3 Mpc.