HI photo-ionization rate measured by different estimators from the literature as a function of redshift. The magenta square shows the estimated value for the emissivity of AGNs at z ~ 4, assuming a luminosity function of Giallongo et al. (2015) down to M1450 = −18. The gray square indicates the contribution of faint AGNs adopting a luminosity function of Glikman et al. (2011). The green and violet squares indicate the AGN emissivity assuming a luminosity function of Parsa et al. (2018) and Akiyama et al. (2018), respectively. A LyC escape fraction of 75% has been assumed. The UVB derived by FG08 has been shown with filled blue circles, while we represent with open blue circles the same data of FG08 rescaled with a different temperature-density relation, as described in Calverley et al. (2011).
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