Cloudy predictions for plane parallel slabs with log(N_H/cm⁻²) = 20.5, photoionized by a single quasar with the characteristic SED of QSO1. The slabs – characterized by uniform n_H – are placed at increasing distance from the quasar. Top: column density of H I as a function of distance from the quasar. The horizontal dotted line indicates the threshold between the optically thin and thick regimes. For n_H = 0.1 cm⁻³ the quasar is able to keep the gas ionized up to ∼200 kpc. Bottom: predicted SB_Lyα as a function of distance from the quasar. The dotted lines indicate Cloudy models which take into account the presence of the z = 3 UVB by Haardt & Madau (2012). The gray shaded region shows the range of observed SB_Lyα.