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5.2 Distinction between FR I and FR II Radio Galaxies

Two decades ago, Fanaroff and Riley (1974) recognized that radio galaxies separate into two distinct luminosity classes, each with its own morphology. As described in Sec. 2, radio emission in the low-luminosity FR Is peaks near the nucleus, while the high-luminosity FR IIs have radio lobes with prominent hot spots and bright outer edges. Jets, when seen, tend to be more collimated in the FR IIs. Examples of both morphological types are shown in Fig. 2.

The luminosity distinction is fairly sharp at 178 MHz, with FR Is and FR IIs lying below and above, respectively, the fiducial luminosity L178 approx 2 x 1025 W Hz-1. The separation is cleanest in the two-dimensional optical-radio luminosity plane, where each class follows a separate, approximately linear, Lr - L0 correlation (Owen and White 1991; Owen and Ledlow 1994), implying the FR I/FR II break depends on optical as well as radio luminosity. At a given radio luminosity, FR Is are more luminous optically than FR IIs although both classes span the same range in optical luminosity overall (Owen and Ledlow 1994).

At higher radio frequencies, the luminosity ranges for each class do overlap by as much as two orders of magnitude, with FR Is having 2.7 GHz luminosities as high as L2.7 ~ 6 x 1026 W Hz-1 while FR IIs have luminosities as low as L2.7 ~ 2 x 1025 W Hz-1 for the 3CR sample (Laing et al. 1983) and even L2.7 ~ 3 x 1024 W Hz-1 for the 2 Jy sample (Morganti et al. 1993).

FR I and FR II galaxies differ systematically in several ways in addition to host galaxy magnitude. At the same radio luminosity or host galaxy magnitude, FR IIs have optical emission lines about an order of magnitude stronger than those of FR Is (Rawlings et al. 1989; Baum and Heckman 1989; Zirbel and Baum 1995). FR Is tend to inhabit moderately rich cluster environments (Prestage and Peacock 1988; Hill and Lilly 1991) in which they are sometimes the first-ranked ellipticals (Owen and Laing 1989), while FR IIs are more isolated.

Classification of radio galaxies is surely more complicated than a simple bifurcation. While FR II morphologies are well defined by their clear outer hotspots and/or bright edges, the FR I class includes many disturbed and atypical radio structures (Parma et al. 1992). At the same time, optical spectra of FR IIs are quite heterogeneous, with some having strong emission lines and others having only very weak, low-excitation emission lines (Hine and Longair 1979; Laing et al. 1994). An alternative classification of radio galaxies by optical spectra might group the low-excitation FR IIs with the (weak-lined) FR Is. The physical connection between FR I and FR II radio galaxies is a clue to the formation of jets and the extraction of energy from the black hole (Baum et al. 1995).

The blurry line between FR types does affect the proposed AGN unification schemes, but only as a perturbation on the basic picture. In terms of statistical analysis, it affects the details of the low-luminosity end of the FR II luminosity function and the high-luminosity end of the FR I luminosity function but has no major impact because what matters in a quantitative analysis (Sec. 6) is the knee of the parent luminosity function.

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