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Astron. Astrophys. 327, 909-920 (1997)

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1. Introduction

Studies of the emission lines in Seyfert galaxies have indicated the presence of two different regions (cf Osterbrock 1989): a region of very high density with large velocity spread - the broad line region (BLR), and a region of low gas density and with a much smaller velocity spread - the narrow line region (NLR). There is also a set of lines of extremely high excitation, the so-called `coronal' lines which includes [Fe VII ] [FORMULA] 6086 @ [Fe X ] [FORMULA] 6374, [Si VI ]1.963 [FORMULA] m and [Si VII ]2.483 [FORMULA] m. Korista & Ferland (1989) showed that photoionization of a "warm" interstellar gas phase could easily account for both the luminosities and the ratios of the coronal iron lines in NGC 4151. Oliva et al. (1994; hereafter OSMM) measured an extensive set of optical and infrared coronal lines in the Circinus Galaxy and showed that the line ratios could be explained better with photoionization than collisional ionization, even when a wide distribution of coronal temperatures was included in the collisional ionization model. Coronal lines are known to be systematically blueshifted (Penston et al. 1984) relative to the systemic velocity in Seyfert galaxies (by 35 [FORMULA] in the case of Circinus, see OSMM), a property which has not been incorporated in any of the models so far.

Korista & Ferland (1989) considered the coronal gas to be of very low density ([FORMULA] cm-3) and to extend up to 2 kpc, much beyond the classical NLR but overlapping the ENLR. From the [Fe XI ] line surface brightness and spatial distribution in the Circinus nucleus, OSMM derived a higher density ([FORMULA] cm-3) and a much smaller size of [FORMULA] pc. Both studies considered that the filling factor of the coronal gas approaches unity and that the gas density is lower than that of the NLR gas. The arguments presented, however, were partly based on the absolute ionising source luminosity, a poorly known quantity at best 1. Furthermore, this view is contradicted in the case of Circinus by the much higher density of [FORMULA] cm-3 inferred by Moorwood et al. (1996: Mo96) from the density sensitive [Ne V ] 24.3 [FORMULA] m/14.3 [FORMULA] m ratio. We propose in this paper that the coronal lines do not originate from a pervading thin medium but from individual gas clouds, as for the NLR. In a previous paper (Binette, Wilson & Storchi-Bergmann 1996, hereafter Paper I), it was proposed that the high excitation lines originated from matter-bounded clouds. In this paper, we argue that the coronal lines originate from a subset of such matter-bounded (MB) clouds which are exposed to a much stronger continuum.

We show in Sect. 2 that radiation pressure can no longer be neglected at the high ionization parameters implied by the coronal lines. We improve on the customary constant gas pressure model by developing a density prescription in the one-dimensional hydrostatic case which considers the distributed force represented by radiative pressure (cf. Appendix A). An interesting property of the resulting density stratification is that the gaseous excitation at high values of the ionization parameter become insensitive to further increase in the intensity of the ionizing radiation. In Sect. 3, we infer the values of the model parameters using available information on the Circinus Galaxy. In Sect. 4 we compare our model results with those of Mo96, while Sect. 5 discusses the implications of the properties of the coronal lines on the ionization-bounded and matter-bounded cloud models of Paper I.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998