Spectral energy distribution of the ionizing radiation that emerges into the IGM during cosmic evolution. We show the contribution from stellar populations in green shades and from AGN in gray. The contribution from stripped stars is highlighted in dark green. They cause the spectra to harden. The dashed lines show spectral slopes of α = −3, −2, and −1, assuming L_ν ∝ ν^α. The panels represent four snapshots during cosmic evolution, taken at z = 1.7, 4.3, 7.0, and 11.7. Following the relation between average metallicity and redshift of Madau & Fragos (2017, see also Sect. 2.1.1), the average metallicity at these times is Z = 0.014, 0.006, 0.002, and 0.0002 (Z_⊙ ≡ 0.014, Asplund et al. 2009). This figure adopts our standard model for the escape fractions, see Table 1.