Evolution of CIGALE models with nebular parameters. This sample of models created by CIGALE is an extract of the entire and much larger one used for the SED fitting (88 × 10⁶ models). Here, in addition to the nebular parameters given in the legend, we fix the SFH (delayed with τ_main = 500 Myrs, and age_main = 100 Myrs), A_V(ISM) = 0.3, T_dust = 40 K, the dust emissivity β = 2.0, and the redshift z = 0. However, to improve the visibility, we offset the spectra by δz = 0.1 on the X axis and 1 dex on the Y axis. The bottom spectra, with log₁₀U = −2 are more representative of the IR spectra emitted by an HII-region dominated galaxy, with a strong [OIII]88.3 μm line (blue-shaded area) detected, for example, in Lyman break galaxies in the early Universe. The top spectra, with log₁₀U = −4, resemble an IR spectra emitted by a PDR-dominated galaxy, with a strong [CII]157.6 μm line (red-shaded area), detected, for example, in DSFGs in the early Universe. A higher gas metallicity amplifies the strength of the metal lines.