7.5. Continuum Reddening
Continuum reddening has been investigated too. This has been clearly seen in some narrow line AGNs but the situation regarding broad line objects is not very clear.
The diversity in the shape of the optical-ultraviolet continuum of Seyfert 1 galaxies and quasars is not very large. This would argue against dust extinction since the effect on the continuum slope is likely to be large, even for a very small amount of dust. Arguments to the contrary have also been raised and there is no conclusive evidence yet. It is however clear that a small proportion of AGNs show a very steep ultraviolet continuum, with a shape that is entirely consistent with dust extinction. It is also clear that the Lyman continuum of some quasars, including low redshift ones (i.e. no significant intergalactic absorption) is too steep to account for the observed emission line strength in these objects.
The dust, if it exists, may have a very different effect on the line and continuum spectra. The dust location is thus of primary importance and further progress in this area must await until this is better understood.
Comprehensive reviews on dust and reddening in AGNs can be found in MacAlpine (1985) and in Osterbrock's book (1989).
Thermal emission and polarization: For model involving dust emission see Barvainis (1987) and Sanders et al. (1989) and references therein. The delayed infrared continuum response in F-9 is described in Clavel, Wamsteker and Glass (1989). There are several infrared polarization studies of quasars, but most of the polarization in such cases is due, probably, to a non-thermal source. A convincing case of dust polarization is described by Rudy and Schmidt (1988). For references on the polarization of Seyfert galaxies see chapter 11.
Internal dust: See Ferland and Netzer (1979), Martin and Ferland (1980) and Cota and Ferland (1988).
Narrow line reddening: Many references are given in MacAlpine (1985). See also De Zotti and Gaskell (1985).
Broad line reddening: See MacAlpine (1985) for references until 1985. Also Ward et al. (1987, use of Paschen and HeII lines), Wamsteker et al. (1990, HeII lines in NGC 5548) and Goodrich (1990, BLR reddening in Seyfert type 1.8-1.9).
Continuum reddening: See references in Lawrence and Elvis (1982) and MacAlpine (1985). Also Sanders et al. (1989).