A guideline to the interpretation of the narrow emission-line spectra of active galactic nuclei

Considering that the narrow emission-line region of active galactic nuclei (AGNs) is composed by clouds moving throughout a dilute gas (<=300 cm^-3^), the emission line spectrum is calculated assuming the coupled effects of shock and photoionization. A comparison between the Hβ line produced by shock and by photoionization shows that these mechanisms could have comparable importance in determining the physical conditions. The contributions to Hβ are of the same order if the cloud velocity is v_0_ <= 300 km s^-1^ and the ionization parameter is U <= 10^4^ (see Fig. 2). For higher velocities the shock effect is still important even for higher values of U (Fig. 2). Theoretical emission-line intensities are given for several lines, for a large range of the input parameters, in order to provide a practical method to analyze observational data. Comparison between theoretical and observed line ratios shows that the presence of shock must be considered when analyzing the emission-line spectrum of AGNS.