2.3. Types of Radio Sources
A simple, but important morphological classification of extended radio structures was made by Fanaroff & Riley (1974), who pointed out that low-power sources tend to be brightest close to their nuclei whereas high-power ones are brightest at their outer extremities. More precisely, if the emission from the central feature is excluded, sources with P1.4 1024.5 WHz-1 in the 3CR sample tend to have their brightness peaks less than half way from the nucleus to the outer edges (FRI) and those with P1.4 1024.5 WHz-1 have the peaks more than half way out (FRII). Further work has shown that this characteristic luminosity is a function of galaxy magnitude, but that the transition remains extremely sharp (Owen 1993; see below).
A slightly more elaborate classification of radio sources is given below. Each example should be treated as a point in a continuum of structures rather than as a representative of a distinct class.
Classical double Cygnus A (Perley, Dreher & Cowan 1984), 3C 179 (Shone, Porcas & Zensus 1985). This term is almost synonymous with the FRII classification. Classical-double sources are brightest at their ends, with compact outer hot-spots and bridges extending back towards the associated galaxy or quasar. Jets, if detected, are almost always of the strong flavor. Central components and jets are more prominent in quasars than in galaxies of the same extended luminosity. In the most powerful sources, the (one-sided) jet and the hot-spot of higher surface brightness are on the same side of the nucleus (Laing 1989). Core-dominated (or flat-spectrum compact) sources appear to be extreme examples of this trend, with very one-sided jets.
Wide-angle tail 3C 465 (Eilek et al. 1984). These sources tend to be found in central cluster galaxies without cooling flows. Their hot-spots are close to the nucleus and are linked to it by strong-flavor jets (O'Donoghue, Owen & Eilek 1990). The jets disrupt at the hot-spots and form extended tails. Some more luminous sources (e.g. 3C 171; Heckman et al. 1984) have similar characteristics.
Bridged twin-jet 3C 296 (Leahy & Perley 1991). These have prominent, two-sided, weak-flavor jets which fade gradually and curve back towards the nucleus to form bridges. The jet bases are almost always one-sided (PKS1333-33; Killeen, Bicknell & Ekers 1986 is an exception). The "naked jets" (Parma et al. 1987) are likely to be examples of this class with extremely weak bridges.
Tailed twin-jet 3C 31 (Strom et al. 1983). This term refers to a minority of the more powerful twin-jet sources. The jets disrupt to form tails extending away from the galaxy, rather than forming bridges.
Narrow-angle tail NGC 1265 (O'Dea & Owen 1986). Versions of the tailed twin-jet sources with C-shaped distortions are found in clusters, associated with fast-moving galaxies. They are referred to as narrow-angle tail or twin-tail sources.
Core-halo/head-tail 0745-191 (Baum & O'Dea 1991); 3C 264 (Gavazzi, Perola & Jaffe 1981). This is an ill-defined class, consisting of sources with kiloparsec-scale, bright central features surrounded by diffuse tails, without obvious collimated jets (although note that several of the breed have turned out to be narrow-angle tail sources when examined in enough detail).
Relaxed double 3C 310 (van Breugel & Fomalont 1984). Similarly, relaxed-double sources have central components and bridges, but no jets. They have luminosities just above the FR break and some, including 3C 310, have lobes with unusually steep and curved radio spectra.