The case for unification.

Abstract

I investigate the issue of whether the various subclasses of radio-loud galaxies are intrinsically the same but have been classified differently mainly due to their being viewed from different directions. Evidence for the two key elements of this popular version of the "unified scheme (US)," relativistic jets and nuclear tori, is updated. The case for the torus opening angle increasing with the radio luminosity of the active galactic nucleus (AGN) is freshly argued. Radio-loud AGN are particularly suited for testing the US, since their structures and polarization properties on different scales, as well as their overall radio sizes, provide useful statistical indicators of the relative orientations of their various subclasses. I summarize recent attempts to bring under a single conceptual framework the USs developed for radio-moderate [Fanaroff-Riley type I (FRI)] and radio-powerful (FRII) AGN. By focusing on FRII radio sources, I critically examine the recent claims of conflict with the US, based on the statistics of radio-size measurements for large, presumably orientation-independent, samples with essentially complete optical identifications. Possible ways of reconciling these results, and also the ones based on very-long-baseline radio interferometry polarimetric observations, with the US are pointed out. By incorporating a highly plausible temporal evolution of radio source properties into the US, I outline a scenario that allows the median linear size of quasars to approach, or even exceed, that of radio galaxies, as samples with decreasing radio luminosity are observed. Thus, even though a number of issues remain to be fully resolved, the scope of unified models continues to expand.