4. HYMORS: a new observational clue to the FR dichotomy
As mentioned in Sect. 1, the remarkable differences between a wide range of characteristics of the FR I and FR II sources, as summarized in Sect. 2, have led several authors to the viewpoint which ties the origin of these differences to the properties of the central engine itself (Sects. 3.2-3.4). It is clearly important to confront this somewhat radical stand with any discriminating observational results available. One possible strategy is to look for double radio sources whose radio structures on the two sides of the nucleus exhibit different Fanaroff-Riley morphologies. Even a few examples of such clearly hybrid morphology double radio sources would call into question models that attribute the FR dichotomy to the properties of the central engine, since in a given double source both radio lobes are presumably caused by a single central engine. On the other hand, such a hybrid morphology may be readily accommodated within a scenario where the ambient media on the two sides of the nucleus have sufficiently dissimilar properties so as to impose different fates upon the two jets emanating from the nucleus.
Following the above reasoning, we have carried out a substantial, though certainly not exhaustive, search of the published literature and have located several cases of HYMORS , which illustrate our point. Below we briefly comment on these individual examples, whose basic properties are summarized in Table 1.
Table 1. Properties of the HYMORS.
0131-367 (NGC 612, z = 0.029): This bright SO galaxy with a prominent dust-lane is the host of a prominent double radio source (Ekers et al. 1978). The hybrid nature of its radio structure is evident from its 5 GHz VLA map which shows a weak core flanked by two radio lobes; the eastern lobe has a bright hotspot near its outer edge (FR II type), whereas the western lobe exhibits a jet-like structure which widens steadily and fades into a diffuse radio plume (FR I) (Fig. 1a; Morganti et al. 1993).
0521-364 (z = 0.055): This well-known blazar, found to be a source of rays above 100 GeV (Thompson et al. 1995), is another fine example of hybrid radio morphology. It consists of a radio/optical synchrotron jet which does not terminate in a hot spot, and a bright radio hot spot on the counter-jet (SE) side, all embedded in a radio halo (Fig. 1b; Keel 1986).
1004+130 (4C+13.41, z = 0.240): The hybrid morphological nature of this quasar is apparent from its 5 GHz VLA map made by Fomalont (1982; Fig. 1c). The radio lobe westward of the bright nuclear core is strongly edge-brightened, typical of FR II sources. In contrast, the eastern lobe is clearly edge-darkened (FR I type), and its structure is dominated by a jet which progressively fades away from the nucleus.
1452-517 (z = 0.016): A 843 MHz MOST map of this giant radio galaxy, made by Jones (1986), is shown in Fig. 1d (see, also, Jones & McAdam 1992). Whilst the N lobe is edge-brightened (FR II type), the other lobe is edge-darkened (FR I) (Jones 1986). A higher resolution map is needed to ascertain if the elongated radio feature seen in the S lobe is indeed a jet. An alternative possibility in the context of such giants is that the emission peak recessed from the outer edge could be the current working surface of a rejuvenated jet which had already reached a much greater extent but was cut off by the instabilities which are particularly likely to afflict such giant, old, radio sources (e.g., Hooda et al. 1994).
1726-038 (4C-03.64): This double source is a good example of a HYMORS. As seen from a recent 4.9 GHz VLA map (Fig. 1e; Jackson et al. 1999), the extremity of the NE lobe is marked by a bright hot spot (FR II type). In contrast, the SW side of the nucleus exhibits a prominent jet which progressively fades away outwards, a standard FR I pattern. Unfortunately, the authors provide no redshift measurement; using the digitized POSS we have attempted to obtain a crude estimate of the redshift. We tentatively identify the source with an elliptical galaxy of approximately 10 arcsec extent. This angular size of the host galaxy suggests a redshift of , taking the intrinsic diameter to be 10 kpc and a Hubble constant of = 75 km s-1 Mpc-1.
2007+777 (z = 0.342): This moderately distant, core dominated BL Lacertae object has been mapped with the VLA at 1.5 GHz by Murphy et al. (1993). As seen from Fig. 1f, the eastern side of the nucleus exhibits a prominent hot spot, whereas the western side is marked by a jet which gradually fades into oblivion at about 10 arcsec from the nucleus.
Some high redshift examples of HYMORS can be identified from the quasar sample imaged by Lonsdale et al. (1993), using the VLA at 5 and 15 GHz. 0038-019 (4C -02.04; z = 1.690) is a good example of hybrid morphology, with a hot-spot in the N lobe and a southward jet without a terminal hot-spot. 1258+404 (3C 280.1, z = 1.659) has a hot-spot on the NW and only a jet to the SE. 1323+655 (4C 65.15, z = 1.618) is another possible HYMORS, with a compact hot-spot to the NE and a bent jet on the SW. The peculiar structures of the first and third of these sources were noted by Lonsdale et al. (1993).
Among the AGN associated with disk galaxies, a HYMORS-like source has recently been mapped in the Seyfert galaxy, Mrk 3 (UGC 3426). Mrk 3 has an extremely low level of nuclear radio activity and its overall radio size is only 0.5 kpc (Kukula et al. 1999). The E jet shows knots close to the nucleus and fades outwards in an FR I pattern, while the W jet terminates in a typical FR II hot spot and lobe.
In addition, 0905-353, which was mapped at low-resolution by Jones & McAdam (1992), is another possible HYMORS. We plan to employ the high resolution and sensitivity of the the Giant Metrewave Radio Telescope to ascertain if this object indeed has a hybrid morphology.
Although Fanaroff & Riley (1974) classified radio sources solely in terms of whether the separation of points of peak intensity were less than (FR I) or greater than (FR II) half of the largest size of the source, more subtle classification schemes have since been proposed. Probably the most widely adopted of these separates double sources into categories depending upon: (a) whether the extended emission is best described as plume-like or bridge-like; and (b) whether the lobes possess compact features or not, and if they do, whether the compact emission is dominated by hotspots, weak jets or strong jets (Leahy 1993, 2000; Laing 1993). Since almost all FR II sources would fall into a the category which is dominated by bridges on large scales and hotspots on small scales, this scheme mainly serves to stress the wide range of types commonly clubbed together in the FR I fold, a few of which might be called `FR 1.5' (Leahy 2000).
Other types of structures intermediate between FR I and FR II morphologies have been noted earlier (Zirbel & Baum 1995and references therein). Owen & Laing (1989) discuss a small group of "Fat Doubles", with bright outer rings and rounded lobes, which they argue are best considered as FR I/II transitional sources. We would not consider any of these as an example of a HYMORS. In their list of some 334 sources, for which they had good enough maps to classify 212, Zirbel & Baum (1995) list only 7 sources as being `FR I/II' (in their paper this means different categorizations had appeared in the literature) and only 3 as being `Transitional'. Published maps could be located for 7 of these 10; we would call four of these clearly FR II, one clearly FR I, one (3C 17) as being very confused, perhaps involving a chance superposition of an FR II and an FR I (Morganti et al. 1993), and only one case (3C 15) that might legitimately be transitional; however, it did not fit our definition of a HYMORS. Nor does PKS 1313-33, a source Morganti et al. (1993) call transitional. In addition, transitional type sources housing plumes and tails along with well collimated jets and weak hot spots, have been seen in three intermediate power radio galaxies (Capetti et al. 1995) and in some low luminosity radio sources (Parma et al. 1987); again, none of these are good examples of HYMORS. Thus, while the sobriquets `FR 1.5' and `FR I/II' have appeared occasionally in the literature, their meanings are imprecise, and we consider ourselves justified in introducing the more specific term HYMORS to describe sources with definite FR I morphology on one side and distinct FR II morphology on the other.
© European Southern Observatory (ESO) 2000
Online publication: December 11, 2000