Origins of the lines. The intrinsic emissivity of selected emission lines in the photoionized Cloudy model, for n_H = 10¹⁰ cm⁻³, and N_H = 10²⁵ cm⁻². Emissivities are expressed in arbitrary units of energy per unit time per unit volume. The ionization parameter is log₁₀U = −3. The hydrogen Balmer and helium lines are intrinsically strong in the inner layers due to recombination emission, although the opacity of Hα and Hβ is high, which leads to scattering effects. [Fe II] is mostly emitted in a warm layer with T_e = 6000–7500 K, which is shielded from the far-UV ionizing radiation.