Left panel: fraction of AGN with L_H > 10⁴² erg s^-1 in groups and small clusters with velocity dispersion 350 < σ < 700 km s^-1. Green symbols are structures from the present work, blue symbols are from Arnold et al. (2009), black symbols from Martini et al. (2009), the red symbol from Eastman et al. (2007). Right panel: the same for clusters with σ > 700 km s^-1. In both plots, to determine the upper limits and to estimate the uncertainties on the fractions we used the low number statistics estimators (at 1σ) by Gehrels (1986). The red solid lines are the predictions from the Millenium simulation (Guo et al. 2011), the black solid and dash-dotted lines are the nominal and maximal predictions using the Menci et al. (2006) model, see text for more details.