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The previous two sections described the abundant evidence for strongly anisotropic radiation from radio-loud AGN. Their appearance therefore depends strongly on orientation, and unification schemes are inevitable. The light from some AGN must be directed toward the observer, whether by relativistic beaming or by obscuration, and the remaining misaligned AGN must constitute the so-called ``parent population.'' (8) The known properties of radio-loud AGN - radio galaxies, quasars, and BL Lac objects - are key to identifying the correct parent and beamed populations.

Here we describe the basic unification schemes that have been proposed to date (Sec. 5.1), and how the isotropic properties of radio-loud AGN support or undermine these schemes. Both FR I and FR II radio galaxies tend to have pairs of jets more or less in the plane of the sky and thus are candidates for misoriented blazars. We discuss the FR I/FR II distinction (Sec. 5.2) and then pursue the two most viable unification schemes, linking quasars with FR II galaxies (Sec. 5.3) and BL Lac objects with FR I galaxies (Sec. 5.4). Specifically, we summarize the principal matching criteria, which include the isotropic properties of extended radio power and morphologies, optical narrow line emission, far-infrared continuum emission, host galaxies, environments external to the AGN, and evolution over cosmic time. Finally, we argue that statistical treatment of proper unbiased samples is not currently possible (Sec. 5.5) and we review the method for incorporating the selection effects introduced by relativistic beaming (Sec. 5.6).

We note that all of the comparisons of FR II galaxies and radio-loud quasars ignored the existence of a separate class of FR IIs with low-excitation optical emission lines (Hine and Longair 1979; Laing et al. 1994). Since the low-excitation FR IIs tend to have lower radio luminosities than the high-excitation FR IIs (Laing et al. 1995), as well as more complex radio morphologies (Laing et al. 1994), the comparisons of isotropic properties described below could be affected in a systematic way. In particular, the low-excitation FR IIs may be more closely associated with BL Lac objects than with quasars. Re-analyzing existing data in the light of this alternative classification would be very interesting.

5.1 History of Radio-Loud Unification Schemes

The suggestion that radio-loud quasars were an aligned version of radio-quiet quasars (Scheuer and Readhead 1979) was an early attempt to unify quasars, but was ruled out by the lack of strong large-scale, diffuse radio emission in the radio-quiet sources. An alternative suggestion was that flat-spectrum radio quasars were aligned versions of steep-spectrum radio quasars (Orr and Browne 1982). This satisfies the requirement for equivalent large-scale radio structures and has not been ruled out. However, a third suggestion, that the SSRQ and FSRQ are increasingly aligned versions of FR II galaxies (Peacock 1987; Scheuer 1987; Barthel 1989) appears to fit the data better (Padovani and Urry 1992).

Since BL Lac objects are also blazars but have much lower equivalent width lines than FSRQ, it was originally suggested that they could be still more aligned versions of quasars (Blandford and Rees 1978). The intrinsic BL Lac luminosities are much smaller than quasar luminosities (for well defined samples), however, and crude estimates of the number densities of parents required are more consistent with low-luminosity radio galaxies (Schwartz and Ku 1983; Browne 1983; Pérez-Fournon and Biermann 1984), as are quantitative estimates of the number densities and luminosities of BL Lacs and FR I galaxies (Ulrich 1989; Browne 1989; Padovani and Urry 1990; Urry et al. 1991a).

8 There is a difference between what should really be called the parent population, which is to say all intrinsically identical objects regardless of orientation, and the misaligned subclass, which is the parents minus the aligned objects. In most cases involving relativistic beaming, however, the relative number of aligned objects is small and there is little difference between the two. Back.

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