3.1. Liners and AGNs
The detection of weak broad H emission (; Barth et al. 1999b), a point like X-ray source (Komossa et al. 1999), a compact ultraviolet nucleus () and/or a compact variable radio core () in a number of Liners supports the hypothesis that some of them are low-luminosity AGNs. By analogy with the nomenclature established for Seyferts,  suggested to extend the ``type 1" and ``type 2" designation to include Liners (L1 and L2).
The overall optical to near-infrared spectral features of Liners with a compact (r < 200 pc) emission line region are adequately reproduced by photoionization calculations which assume a non stellar ionizing continuum, solar abundances, and ionization parameters in the range ~ 10-3 to 10-4; allowance for variations in the total hydrogen density and in the hardness of the ionizing continuum accounts for the range of observed line intensity ratios (Ho et al. 1993a, b). The existence of a strong correlation between line width and critical density for de-excitation in the narrow-line region of some well studied Liners shows that the range of densities within each of these regions is indeed very large (Ne = 103-107 cm-3) (; ). The weakness of He II emission in Liners is a well known problem and indicates that the continuum illuminating the NLR clouds must contain few photons more energetic than 54 eV, the ionization potential of He+; this problem has not yet been convincingly solved (; ).
The S0 galaxy NGC7213 has broad Balmer lines and is therefore certainly an AGN; its narrow-line spectrum is that of a Liner; the nuclear IR to UV continuum is similar to that of a typical Seyfert 1 (). NGC5252 contains a nuclear emission component which has a Liner spectrum in addition to extranuclear knots having Seyfert 2 spectra (). In both cases, photoionization by a non stellar continuum is the most likely mechanism of excitation, the low-ionization lines indicating that the ionization parameter is lower than in classical Seyferts.
However,  have shown that the distribution of the 6300 / 5007 ratios for Seyfert 2s and Liners is probably bimodal. If Liners were similar to Seyfert 2s, the only difference being a smaller value of the ionization parameter, we would probably expect a continuous distribution of the values of this ratio. We shall see below (sec. 4.5.2) that most Liners may differ drastically from Seyfert 2s by the presence in their center of a geometrically thick rather than thin disk.