2.4. Propagation of Jets and the Types of Radio Source
A good guess is that all radio sources contain strong-flavor jets, since these are observed at the bases of most weak-flavor jets (given sufficient resolution and sensitivity) as well being the only type of jet seen in more powerful sources. Almost all known parsec-scale jets are one-sided (Marscher, these proceedings) and the majority of central components are therefore thought to contain jets of this type. A way of viewing the various types of source described earlier is then as an evolution of the flow, starting with strong-flavor jets, evolving into other high-brightness structure (hot-spots or weak-flavor jets) and finally forming bridges or tails. The "allowed transitions" are sketched in Figure 1 (compare Figure 1 of Leahy 1993, which is an alternative way of presenting the same information). It is not yet clear whether strong-flavor jets can evolve directly into bridges or tails, as it is possible that more detailed observations of core-halo or relaxed-double sources will reveal intervening weak-flavor jets or hot-spots (as was the case for Fornax A: Geldzahler & Fomalont 1984; Fomalont et al. 1989). For the moment, these possibilities are retained in Figure 1.
Figure 1. A schematic representation of the radio-source classification discussed in the text as an evolution from strong-flavor jets through other high-brightness structure to extended lobes. Each radial line represents one of the families of radio source discussed earlier and passes through regions corresponding to the ingredients of Section 2.1 in order of decreasing surface brightness.
The factors which determine the morphology of a radio source have not been disentangled, but the galactic environment and beam power are both clearly important. A particularly interesting recent discovery relating to the environment is that the transition luminosity dividing the FRI and FRII classes is a surprisingly well-defined function of the magnitude of the parent galaxy (Owen 1993; see Figure 2). Rawlings & Saunders (1991) have also argued that the flux of radiation ionizing the narrow-line region and the energy flux of the radio jets are closely correlated. To form an FRII radio source, then, it appears to be necessary to have both a powerful active nucleus and a galaxy which is not too luminous.
Figure 2. A plot of monochromatic radio luminosity at 1.4 GHz against R magnitude of the parent galaxy within the 24.5m isophote (corrected to rest frame). The data are taken from Owen (1993), scaled to H0 = 100 kms-1 Mpc-1. Open circles: FRI, filled circles: FRII. Note the tendency for the transition luminosity between the two morphological classes to increase with the optical brightness of the galaxy.