The origin of the blue continuum in type 2 Seyferts: reflection or star-forming tori?

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Abstract
In the so-called unified Seyfert model, type 2 Seyferts are interpreted as normal Seyfert 1 nuclei whose central regions, containing the ionizing source and the broad-line region, are obscured by an opaque dusty torus. Reflection of the nuclear light by electrons and/or dust in the extranuclear regions nevertheless provides a periscopic view into the hidden active nucleus. In this model, the blue and featureless continuum observed in Seyfert 2s is a mere reflection of the obscured Seyfert 1 ‘non-stellar’ continuum. If this is indeed the case then broad lines associated with the obscured nucleus should also be detected as some level. In this paper we analyse the detectability of reflected broad emission lines and use this as a test of the consistency of the unified model. We investigate two possible explanations for the absence of broad lines in Seyfert 2s: (i) the broad components are too faint in contrast with lines from the narrow-line region and (ii) the broad-line profile is broadened to very large widths on scattering by hot electrons, rendering the reflected line undetectable. We predict the strength and detectability of reflected broad lines in both scenarios. Our results put tight constraints on reflection models. They rule out dust scattering and impose a minimum temperature of 106 K if the scatterers are electrons. The implications of these findings for the polarization and emission from the scattering region are examined. The simplest way to explain the absence of conspicuous broad lines in Seyfert 2s is to admit that most of their blue continuum does not come from reflection of a hidden Seyfert 1, but from some extra source. The possibility that this extra source consists of a population of young stars in the vicinity of the nucleus is discussed. We conclude that many of the problems of the unified model can be solved if the obscuring torus is made up of dust and young stars.

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