Seyfert 2s and Liners

It has been suggested by several authors (see for instance Ferland & Netzer 1983; Shields 1992; Ho et al. 1993a) that in Seyfert 2s, as well as in Liners, the ionized gas is excited by a non-thermal continuum, the only differences being the value of the ionizing parameter which would be $\sim$ 10^-3.5 for Liners, and $\sim$ 10^-2.5 for Seyfert 2s. If this is the case, the discontinuity between Seyfert 2s and Liners is not easily understood. No reliable detection of the Heii line in bona fide Liners has been reported suggesting that there could be a serious problem with the picture of simply reducing U in a standard power-law photoionization model predicting $\lambda$4686/H$\beta$ > 0.15 (Viegas-Aldrovandi & Gruenwald, 1990), as the weakness of Heii indicates that the continuum illuminating the NLR clouds must contain few photons more energetic than 54.4 eV, the ionization potential of He⁺ (Péquignot 1984). Binette et al. (1996) proposed that the emission spectrum of Liners is due to ionization-bounded clouds illuminated by a ionization spectrum filtered by matter-bounded clouds hidden from view by obscuring material. In this case, the Heii emission is reduced ($\lambda$4686/H$\beta$ < 0.01). However, a nearly total obscuration of the matter-bounded component must then be invoked in order to keep the emission from Heii at an acceptable low level, a scenario which seems to be rather unlikely to Barth et al. (1996).