Dual-population unification: Why unify ?

The central tenet of the current version of the unified scheme for radio-loud AGN is that the radio emission from the cores of powerful radio sources is highly anisotropic. Observationally, this manifests itself in two apparently exotic physical processes: (i) `superluminal motion' of discrete blobs of radio plasma close to the radio core and (ii) surface brightness temperatures which far exceed the limit set by inverse Compton processes. Both processes can be elegantly explained by a geometrical effect, relativistic Doppler boosting, which enhances the apparent speed and flux density of the approaching radio plasma (Rees 1967). As the radio jets are collimated, with opening angles , relativistic boosting only occurs for sources with their radio-jet axis in close alignment to our line-of-sight, so that most sources are observed `unboosted' in a randomly-oriented sample.

In addition to the radio jet orientation there is a second orientation-dependent feature invoked to explain broad and narrow optical/UV emission line features observed in powerful FRIIs. In these sources a dusty torus shields/reveals the central regions of the source, again depending upon the orientation of the source with respect to the observer. This is the same unification mechanism invoked to unify Seyfert I and II galaxies (Antonucci & Miller 1985). The torus opening angle has been determined to be

from the relative numbers of broad- and narrow-line FRIIs (Laing et al. 1994).

The `dual population' unified scheme adopted by Wall & Jackson (1997) is illustrated in Figure 1. In this scenario, FRII radio galaxies are the parent population of all quasars and some BL Lac-type sources whilst FRI radio galaxies being the parent population of the remainder of the BL Lac-type sources. This differs from the more straightforward FRII-quasar, FRI-BL Lac unified scheme of Urry & Padovani (1995) in that we consider the two optical/UV spectral classes of FRIIs: in particular, low-excitation FRIIs show only weak narrow optical/UV emission lines or none at all and hence their beamed counterparts must appear as BL Lac-type sources rather than quasars. The known correlation between emission-line intensity and radio luminosity (Hine & Longair 1979) suggests that the split of the FRII population into low and high-excitation type is a function of intrinsic radio power (Laing et al. 1994; Barthel 1994). There are about equal numbers of high and low-excitation FRIIs at the low-power end of the FRII class (

W Hz^-1 sr^-1), rising to almost exclusively high-excitation FRIIs at the highest radio powers (

W Hz^-1 sr^-1).

© Copyright Astronomical Society of Australia 1997