Figure 8: Fraction of type 2 AGN as a function of X-ray luminosity (from Hasinger [135]). The solid symbols show hard X-ray-selected AGN (blue and black symbols illustrate different subsamples in terms of redshift completeness). The red-dotted symbols show optically selected AGN (from Simpson [131]). The magenta line shows the type 2 fraction inferred from the dust covering factor obtained through near/mid-IR observations (from Maiolino et al. [139], note that these include also Compton-thick AGN, which are not present in X-ray samples). The green-dotted line shows the expected fraction of Compton-thin type 2 AGN according to the X-ray background synthesis models given in Gilli et al. [140].