Number of photons emitted above the energy E as a function of E, for different incident radiation fields. The ionization thresholds of key ions discussed in this paper are presented by the vertical dashed lines. Panel a: POPSTAR models at an age of 1 Myr color-coded by stellar metallicity, from Z = 0.01 Z_⊙ (solid blue) to 2 Z_⊙ (solid red). In addition, a BPASS 300bin model at an age of 1 Myr with a metallicity of Z = 0.05 Z_⊙ is presented (dotted cyan). Panel b: AGN models color-coded by power-law index, from α = −1.2 (blue) to α = −2.0 (red). The POPSTAR model at an age of 1 Myr with a metallicity of Z = 0.2 Z_⊙ is shown by the gray curve for comparison (same for the rest of the panels). Panel c: POPSTAR models at an age of 1 Myr combined with AGN with an α = −1.6. Two cases of the contribution of the AGN to the total number of ionizing photons are presented; f_AGN = 0.03 (solid) and 0.1 (dotted). The color-code is the same as the panel a. Panel d: blackbody models with a high effective temperature of T_eff = 6 × 10⁴ K (solid red), 8 × 10⁴ K (solid yellow), and 1 × 10⁵ K (solid green). In addition, a PopIII model of Raiter et al. (2010) corresponding to zero-age with a Salpeter (1955) IMF 1–100M_⊙ is presented (dotted blue).