TOWARD UNDERSTANDING THE FANAROFF-RILEY DICHOTOMY IN RADIO-SOURCE MORPHOLOGY AND POWER

In Paper I we presented the results of a study of the interrelationshi ps between host galaxy magnitude, optical line luminosity, and radio l uminosity in a large sample of Fanaroff-Riley classes 1 and 2 (FR 1 an d FR 2) radio galaxies. We report several important differences betwee n the FR 1 and FR 2 radio galaxies. At the same host galaxy magnitude or radio luminosity, the FR 2's produce substantially more optical lin e emission (by roughly an order of magnitude or more) than do FR l's. Similarly, FR 2 sources produce orders of magnitude more line luminosi ty than do radio-quiet galaxies of the same optical magnitude, while F R 1 sources and radio-quiet galaxies of the same optical magnitude pro duce similar line luminosities. Combining these results with previous results from the literature, we conclude that while the emission-line gas in the FR 2's is indeed photoionized by a nuclear UV continuum sou rce from the AGN, the emission-line gas in the FR 1's may be energized predominantly by processes associated with the host galaxy itself. Th e apparent lack of a strong UV continuum source from the central engin e in FR 1 sources can be understood in two different ways. In the firs t scenario, FR 1's are much more efficient at covering jet bulk kineti c energy into radio luminosity than FR 2's, such that an FR 1 has a mu ch lower bolometric AGN luminosity (hence nuclear UV continuum source) than does an FR 2 of the same radio luminosity. We discuss the pros a nd cons of this model and conclude that the efficiency differences nee ded between FR 2 and FR 1 radio galaxies are quite large and may lead to difficulties with the interpretation since it would suggest that FR 2 radio source deposit very large amounts of kinetic energy into the ISM Intracluster Medium. However, this interpretation remains viable. Alternatively, it may be that the AGNs in FR 1 sources simply produce far less radiant UV energy than do those in FR 2 sources. That is, FR 1 sources may funnel a higher fraction of the total energy output from the AGNs into jet kinetic energy versus radiant energy than do FR 2 s ources. If this interpretation is correct, then this suggests that the re is a fundamental difference in the central engine and/or in the imm ediate ''accretion region'' around the engine in FR 1 and FR 2 radio g alaxies. We note also the absence of FR 1 sources with nuclear broad l ine regions and suggest that the absence of the BLR is tied to the abs ence of the ''isotropic'' nuclear UV continuum source in FR 1 sources. We put forth the possibility that the FR 1/FR 2 dichotomy (i.e., the observed differences in the properties of low- and high-power radio so urces) is due to qualitative differences in the structural properties of the central engines in these two types of sources. Following early work by Rees et al. (1982), we suggest the possibility that FR 1 sourc es are produced when the central engine is fed at a lower accretion ra te, leading to the creation of a source in which the ratio of radiant to jet bulk kinetic energy is low, while FR 2 sources are produced whe n the central engine is fed at a higher accretion rate, causing the ce ntral engine to deposit a higher fraction of its energy in radiant ene rgy. We further suggest the possibility that associated differences in the spin properties of the central black hole between FR 1 (lower spi n) and FR 2 (higher spin) sources may be responsible for the different collimation properties and Mach numbers of the jets produced by these two types of radio-loud galaxies. This scenario, although currently c learly speculative, is nicely consistent with our current picture of t he triggering, feeding, environments, and evolution of powerful radio galaxies. This model allows for evolution of these properties with tim e-for example, the mass accretion rate and BH spin may decline with ti me causing an FR 2 radio source or quasar to evolve into a FR 1 radio source.