1997A&A...323...71C

The physical conditions which characterize individual Liners are obtained by fitting the observational spectra of the Ho et al. (1993ApJ...417...63H) sample by model calculations. Composite models which consistently account both for shocks, accompanying the radial outward motion of the gaseous clouds, and a photoionizing radiation flux are used. Power-law or black body radiation is considered, depending on the characteristics of the line spectra. The SUMA code is used. The contributions of the different ionization mechanisms to the spectrum emitted by each object are calculated and compared with the observations. The most significant line ratios of Liner spectra are analyzed to show the relative importance of photoionization by radiation from the active center or from starbursts, of photoionization by diffuse radiation, and of collisional ionization by shocks. Model results show that [Ne V] lines can be strong due to collisional ionization and/or photoionization. Lines from the II and III ionization levels are usually due to photoionization, but a strong contribution by collisional ionization appears in case of high densities and/or of a strong shock. Neutral lines are enhanced by diffuse radiation. The contribution of high density gas to the line intensities in Liner spectra is very small, indicating that the high density region, located between the NLR and the BLR, is also small. The results show that shocks play an important role, even if a photoionizing flux is present. On average, higher than cosmic N/H and lower S/H are indicated. The physical characteristics of Seyfert galaxies and of starburst galaxies are confirmed. The satisfactory results obtained interpreting the heterogeneous observational sample by SUMA strengthen the hypothesis that shock signature in AGN spectra is due to cloud motions in galaxies.