Figure 19: Number density of X-ray-selected AGN as a function of redshift for sources with  erg s−1, as published by Brusa et al. [83]. Lines show AGN luminosity functions [39, 79, 8486] while individual measurements are shown by green circles [87] and black squares [86]. The red star at was obtained from the optical luminosity function assuming no evolution in . The black-shaded area shows the observations of optically bright SDSS QSOs [88].