Limiting SB and density quantities as a function of redshift. The top panel shows the limiting SB of Lyα in the mirror assumption, where 65% of ionising photons in the UVB are reprocessed into Lyα photons (see text for details) and the bottom panel shows the self-shielding critical density contrast Δ_crit. The two different lines correspond to UVBs of Haardt & Madau (2012, HM12) and Puchwein et al. (2019, P19). Above z > 6, where the line is dashed, the P19 limits are not representative of ionised bubbles during patchy reionisation because the impact of neutral regions on the effective opacity to hydrogen ionising photons is included in the modelling (see P19) and hence a neutral hydrogen-weighted average over both neutral and ionised regions is computed in that model. A redshift of z = 4.8 is highlighted by the dashed line.