Space density of Compton-thick AGN samples with different intrinsic 2–10 keV luminosities compared with the XRB model predictions (Gilli et al. 2007). Datapoints have to be compared with the corresponding color curves; luminosity thresholds are as labeled. References to the observed densities, sorted by increasing luminosity threshold, and by increasing redshift for the same luminosity threshold, are as follows: L11: Luo et al. (2011); D07: Daddi et al. (2007b); V14: this work; F09: Fiore et al. (2009); A08: Alexander et al. (2008); V10: Vignali et al. (2010); P06: Polletta et al. (2006); B10: Bauer et al. (2010).