8.2.6 Extended Continuum Emission in Type 2 AGN
Spectropolarimetry has indicated an interesting complication in a number of Type 2 AGN; specifically, the scattered broad-line flux is sometimes more polarized than the continuum (Tran 1995). This implies that in addition to the nuclear power-law continuum source there must be an extended continuum source in the scattering region which dilutes the polarized flux of the scattered nuclear continuum. The in situ extended featureless continuum must be similar to the nuclear power law, and in some ways the match between the nuclear and extended properties is a bit mysterious. The details of the spectral shapes depend on very high signal-to-noise spectropolarimetric data, however, available only for a few of the brightest sources, and so remain to be determined for the Type 2 AGN as a class.
The presence of an extended optical continuum component can also be inferred from the uniformity of H equivalent widths in AGN types from Seyferts to radio-loud Type 2 AGN to quasars (Binette et al. 1993). According to unification, the intensity of the narrow H line should be relatively orientation independent while the broad line and the nuclear continuum are not, meaning the equivalent width should differ strongly between Type 1 and Type 2 AGN. The lack of a difference can be explained if appreciable continuum emission is generated in or near near the line-emitting cloud, roughly in proportion to the incident photoionizing continuum, in which case the equivalent width will not depend strongly on orientation.
Locally-generated continuum, such as shock-excited emission from a jet interacting with the interstellar medium (Sutherland et al. 1993), could also explain the similar reddening seen in lines and continuum (Binette et al. 1993), as well as the association of optical emission line gas with extended radio structure (Sec. 3.1). Extended optical/UV continuum associated with emission-line regions has in fact been seen in some nearby Seyfert 2 galaxies (Pogge and De Robertis 1993) and in Cygnus A (Pierce and Stockton 1986; Antonucci et al. 1994). To first order, an extended continuum source should mitigate the selection effects of beaming in the optical band; the degree to which this is the case depends critically on its luminosity relative to the nuclear continuum.