Astron. Astrophys. 336, 823-828 (1998)

8. Comparison with thick tori model predictions

Several torus models have been developped so far to explain the obscuration of the BLR and UV/X-ray continuum sources from some lines of sight (AGN unification scheme).

Pier & Krolik (1992a, 1992b, 1993) propose a thick parsec scale uniform density annular ring of dust, illuminated by a central point source. The dust can be heated up to T_eff, the effective temperature of the nuclear radiation on the inner edge of the torus. They investigate models with 500 KT2000 K. This kind of model could explain the unresolved core observed in NGC 7469. What about the emission of the extended dust component ? In the case of NGC 1068, a bright Seyfert 2 nucleus, these authors explain the 1"-2" extended infrared emission partly by reflected radiation from the torus and partly by dust in the NLR. In the case of NGC 7469 this interpretation seems difficult to conciliate with our observations, because the dust temperature is shown to decrease monotonically with the distance from the central engine, suggesting the latter to be responsible for the dust heating.

Efstathiou & Rowan-Robinson (1995) propose a model with a very thick tapered disk following a density distribution. They assume the melting temperature of all dust grains to be identical (1000 K), but they note that the radius at which each grain type in the mixture reaches this temperature is different. In the case of NGC 1068, Efstathiou et al. (1995) have shown that the torus emission alone cannot account for the entire infrared emission. They attribute the excess infrared emission to a component of optically thin dust distributed as within the NLR region. This model could apply as well in the case of NGC 7469, as we observe an unresolved core and an extended, temperature-decreasing component.

Granato & Danese (1994) and Granato et al. (1996, 1997) developped a simple thick (30) torus model extended over several hundreds of parsecs. To keep the number of free model parameters to a minimum, they have adopted a dust density distribution constant with radial distance from the central source. This choice is clearly not compatible with our observations in the case of NGC 7469. Nevertheless, they do not rule out the possibility, in case of smaller values of optical depth (=1.5), that a more concentrated density distribution would be needed (following a density distribution).

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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