Histogram of Lyα luminosities of recombination (panel a) and collisional excitation (panel b) processes in the same region as shown in Fig. 5 (a narrowband with Δλ_obs = 8.75 Å, equivalent to ∼2.7 h⁻¹ cMpc) in a simulation snapshot at z = 4.8 in phase space. The colour represents the total luminosity in the simulation per histogram bin. The horizontal dashed line corresponds to the lower limit above which the fitting function of Scholz et al. (1990) and Scholz & Walters (1991) for collisionally excited Lyα emission is valid; the upper limit lies above the plotted range. The vertical dotted line shows the critical self-shielding density threshold at this redshift for the HM12 UVB (from Eq. (13) in Rahmati et al. 2013). Densities above the threshold are also more strongly affected by modelling uncertainties.