AGN fraction (f_AGN) as a function of velocity dispersion of galaxy clusters. Left and right panels are for early- and late-type galaxies, respectively. f_AGN is computed in the sample of galaxies [C1(bottom) − C3(top)] within the virial radius of the clusters (R ≤ r_200,cl). Clusters with f_AGN = 0 are indicated by arrows at the positions of upper limits. Horizontal dashed line indicates the mean f_AGN using all the cluster galaxies in the sample, and dotted lines are its errors. f_AGN in the sample of field galaxies (R > r_200,cl) and of BCGs is shown to the left and right of each panel, respectively.