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Astron. Astrophys. 355, L31-L33 (2000)

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2. A modification of the unification model

The proposed modification of the unification model is illustrated in Fig. 1. The basic properties are as follows:

  • In all Seyferts there are dust lanes on scales of hundred of parsecs, as observed by Malkan et al. (1998). These lanes have column densities of the order of 1022-1023 cm-2 at most, otherwise the mass involved would be too large. The fact that the dust content of Seyfert 2s appears, on average, to be greater than that of Seyfert 1s may be related to the more disturbed morphology of the Seyfert 2s host galaxies, possibly as a result or a recent interaction with another galaxy.

  • Not all Seyferts have the torus (or, at least, not all have a torus with a large covering factor). Again, there may be a greater chance of producing a torus in Seyfert 2s, as they are more disturbed and with a larger overall dust content.

[FIGURE] Fig. 1. The proposed unification model (see text for details).

There are, therefore, three different possibilities:

  • The sources observed through a dust lane (but outside the torus) are the Compton-thin (in X-ray terminology) or intermediate (in optical terminology) Seyferts.

  • The sources observed through the torus are the strict Seyfert 2s (most of them Compton-thick, using the X-ray terminology).

  • If the line-of-sight to the nucleus is free of any absorber, the source is a Seyfert 1. Of course, it is more likely (but not necessary) that a source is observed as Seyfert 1 when the torus is not present.

It is worth remarking that the fraction of sources with the thick torus must be fairly large, as Compton-thick sources account for at least half of the total number of obscured Seyferts (Risaliti et al. 1999).

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© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000