The unification of radio galaxies and quasars: implications of the radio sizes and number densities

Abstract

We examine the recent suggestion that the relative counts and median radio sizes of the moderately distant quasars and radio galaxies in the 3CR complete sample are inconsistent with the simple unification scheme, according to which radio galaxies appear as quasars to those observers whose lines of sight allow a direct view of the nuclear region through the openings in the optically thick torus surrounding the nucleus. The axis of such roughly bi-conical openings is generally assumed to be perfectly aligned with the main axis of associated radio source. We argue that, by relaxing this assumption marginally, the above-mentioned inconsistency with the unification scheme can be largely resolved, and some other statistical correlations involving the radio sizes of quasars can be brought into closer agreement with the corresponding trends displayed by radio galaxies. The possible relevance of such misalignments to the formation of the `compact double' and the gigahertz-peaked- spectrum' (GPS) radio sources is also pointed out.