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Attention is given to the spectra of low-ionization nuclear emission-line regions (Liners) of galaxies as compared to other active galactic nuclei. Results are presented for extensive photoionization model calculations which are employed for the investigation of line ratio diagrams. The diagrams show that broad-line objects, Seyfert 2 galaxies and Liners form a continuous smooth sequence of decreasing ionization parameter. The data suggest a common source for line excitation in all of them. The results indicate that: (1) photoionization by a nonthermal source can explain the observed spectra of all active nuclei; (2) several weak lines differentiate between photoionization and shock-heated models; and (3) the nonthermal ionizing continuum observed in several Liners is consistent with observed line fluxes.
Ferland et al. (Sat,) studied this question.