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Astron. Astrophys. 333, 877-881 (1998)

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3. Discussion and conclusions

The ionization mechanism within Liners is still a subject of debate, mainly because their emission line spectrum can be reasonably reproduced by very different models, based on shock excitation, hot stars, or non-stellar photoionization. It has been shown that Liners and Seyfert 2 galaxies could be photoionized by the same non-thermal continuum, with a lower ionization parameter for Liners. The ionization parameter U (U = [FORMULA] (4 [FORMULA])) is the number of available ionizing photons by hydrogen atom (Q is the number of Lyman continuum photons emitted by the source per second, r the distance of the emitting gas from the source, [FORMULA] the electron density of the cloud and c the speed of light). Ferland & Netzer (1983) showed that Seyfert 2s correspond to [FORMULA] 3 10-3 and Liners to [FORMULA] 3 [FORMULA].

Halpern & Steiner (1983) suggested that dilution of the input continuum could be obtained if cold clouds with column density [FORMULA] = 10 [FORMULA] cm-2, typical of broad-line clouds in Seyfert 1 galaxies, were covering a fraction f of the continuum as seen from the narrow-line region. The effect of covering is almost equal to a decrease in the ionization parameter U by a factor (1 [FORMULA]). Liner spectra would be obtained for f [FORMULA] 0.90-0.98. Schultz & Fritsch (1994) and Binette et al. (1996) have proposed very similar models to produce Liner spectra in which an average AGN continuum is distorted or "filtered" by matter-bounded clouds, hidden from view by obscuring material. An intervening ionized cloud with log [FORMULA] = 20 would reduce the ionization parameter by a factor of [FORMULA] 10. Binette et al. argue that, in these models, the predicted He ii [FORMULA] 4686 [FORMULA] H [FORMULA] ratio is [FORMULA]  0.01, in agreement with the fact that no reliable detection of He ii has been reported in Liners, while if U is simply reduced, as proposed in the Halpern & Steiner model, without altering the shape of the ionizing spectrum, the expected [FORMULA] 4686 [FORMULA] H [FORMULA] ratio is [FORMULA] 0.15. In the case of NGC 5252, the He ii line could not be firmly detected on the HST nucleus spectrum; nevertheless, an upper limit to the relative flux of this line to H [FORMULA] can be estimated at [FORMULA] 0.10, which does not allow us to decide between the two models.

In NGC 5252, a Seyfert 2 and a Liner are simultaneously present. Both near-IR and X-ray observations reveal the presence of an obscured non-stellar nuclear source. This source is most probably responsible for the ionization of the Seyfert 2 nebulosity. We suggest that the Liner is also ionized by this nuclear source, attenuated by an intervening matter-bounded cloud hidden from view by the same material which obscures the nuclear source; the "filtering" material would only partially cover the ionizing source, the Seyfert 2 clouds "seeing" directly this source without any intervening matter.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998