5. Concluding remarks
Our main objective in this study was to highlight the existence of double radio sources whose structures are characterized by a hybrid morphology in terms of the Fanaroff-Riley classification scheme. Table 1 summarizes the basic radio and optical data for six good examples of such HYMORS (Sect. 4). It is seen that the hybrid radio morphology can be associated with all three major classes of radio powerful AGNs (i.e., galaxies, quasars and BL Lac objects), and with radio sources extending from galactic up to megaparsec dimensions. Although the radio powers of these HYMORS are below or around the critical value for the FR I/FR II transition, this probably is due to the modest redshifts of these sources; their counterparts may well exist at high redshifts (, Sect. 4). From Fig. 1 it may also be noted that in some of the cases the FR I classification of a radio lobe is based on the detection of a radio jet without a terminal hot spot; in the other cases a diffuse radio lobe without a hotspot (FR I type) is found associated with a jet. We note that since the missing hotspot in HYMORS is often on the side with the jet, the morphological asymmetry cannot be explained by postulating even an implausibly strong Doppler boosting of the hotspot radiation.
Because we have examined very heterogeneous samples of radio maps in order to produce our set of HYMORS, it is not possible to give an accurate estimate of their frequency. We examined roughly 1000 radio maps overall to come up with the 6 good examples highlighted above. Among relatively well defined samples, we found: 1 HYMORS out of 181 3CRR galaxies considered by Laing et al. (1983); 1 case among the 150 4C maps presented by Jackson et al. 1999; and 0 cases in the 98 maps of steep spectrum 4C sources considered by Rhee et al. (1996). These are consistent with very low rates of less than about . The higher redshift sample of Lonsdale et al. (1993), however, yielded 3 HYMORS out of 70 sources.
Although rare, HYMORS can serve as a very useful discriminator between the wide range of theories that have been put forward to explain the origin of the FR dichotomy. The mere existence of HYMORS seems to disfavor the class of models that posit fundamental differences between the central engines, such as spin or jet composition, as being the dominant cause for the two morphological types (Sects. 1, 3). The scheme of Wang et al. (1992) (Sect. 3.4), where the central engine is argued to be capable of ejecting jets of grossly unequal power, could conceivably be reconciled with the existence of HYMORS. However, even if such an asymmetry could somehow be maintained over most of the source lifetime, there is no significant evidence in our sample of HYMORS for the positively correlated asymmetries in radio luminosities and lobe lengths that this scenario would predict. (If one leaves the standard jet paradigm and considers gravitational slingshot scenarios (e.g., Valtonen & Heinämäki 2000), then some HYMORS would be expected, though the predicted lobe length asymmetry cannot be verified using the present small sample.)
Thus, at least in the case of HYMORS, it appears that some type of jet-medium interaction on kiloparsec scales is playing a crucial role in creating the morphological asymmetry about the nucleus. If the asymmetric jet/medium interaction is such that the jet collimation is quite different on the two sides, the side with poorer collimation, and therefore more rapid slowdown, would lose its hotspot sooner. Earlier, environmental asymmetries have been argued to be important for the Compact Steep Spectrum radio sources (Gopal-Krishna & Wiita 1991; Saikia et al. 1995; Jeyakumar et al. 2000). A more extensive search for HYMORS, allowing a reliable statistical estimate of their frequency and structural asymmetries, is likely to provide a deeper insight into the origin of the Fanaroff-Riley morphological dichotomy.
A recent study appears to underscore the dominant role of accretion in the jet formation. Serjeant et al. (1998) show that for steep spectrum quasar cores, radio and optical outputs are strongly correlated; this implies a close link between the formation of jets and accretion onto the SMBH, improving on the similar argument made by Rawlings & Saunders (1991; see also Falcke & Biermann 1999).
Finally, in the context of HYMORS it may be pertinent to quote Lilly & Prestage (1987): "It is important to stress that the environment must be influencing not just the outer radio lobes, but also nuclear phenomena, such as the strength of the optical emission lines and the properties of the radio jets." We surmise that the observed differences between the properties of the host galaxies of FR I and FR II sources engender the dichotomy by creating different environmental conditions to be encountered by the jets, rather than by producing fundamentally different kinds of central engines.
© European Southern Observatory (ESO) 2000
Online publication: December 11, 2000