Luminosity density. Upper panel: luminosity density at 1.4 GHz calculated for a set of different luminosity ranges of the same width, as denoted in the legend above the figure. The black dots represent the maximum value of each line. Middle panel: luminosity density at 1.4 GHz calculated for a set of progressively increasing luminosity ranges, as denoted in the legend above the figure. Lower panel: luminosity density at 1.4 GHz as a function of redshift for a set of different surveys, denoted in the legend. The data points denote the high-redshift quasar surveys, as described in the text. The uncertainties in the figure follow those in Fig. 8. The shaded area in the plots denote a higher redshift where the LF models are less constrained.