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Astron. Astrophys. 363, 507-516 (2000)

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2. Observational aspects of the FR I/FR II dichotomy

Out of the vast amount of literature now available on this topic, we attempt here to recapitulate some of the prominent distinctions claimed to exist between the two FR classes. It is now well established that: FR II jets on kpc scales are distinctly more asymmetric and better collimated than FR I jets; the magnetic field in a FR II jet remains aligned with the jet along most of its length, while in a FR I jet the magnetic field is predominantly transverse on multi-kpc-scales (e.g. Bridle & Perley 1984). Also, the radio nucleus is more prominent in the FR I sources (e.g. Morganti et al. 1993); however, the difference vanishes if FR I and FR II sources of the same radio luminosity are considered (Zirbel & Baum 1995). VLBI measurements of nuclear jets, which often exhibit superluminal motions, strongly suggest that FR II jets are relativistic on parsec scales, and there are now many cases where FR I jets also appear to flow relativistically on such scales (Giovannini et al. 1995; Bridle 1996; Laing 1999; Biretta et al. 1999; Xu et al. 2000). Doppler boosting can explain the facts that powerful FR II jets appear one-sided while weaker FR I jets exhibit large brightness asymmetries only near their origins, and typically have short, one-sided basal regions (Bridle & Perley 1984; Parma et al. 1987). Moreover, the Laing-Garrington (e.g., Garrington et al. 1988) depolarization asymmetry is exhibited by the lobes of some FR I sources as well as by the FR II sources in which it was discovered (Parma et al. 1987; Laing et al. 1999). On the other hand, while the evidence for FR II jets retaining their relativistic bulk velocities up to the multi-kpc scale is very strong, with the brighter large scale jet always seen on the same side as the nuclear jet and towards the less depolarized radio lobe (Scheuer 1987; Garrington et al. 1988; Bridle 1996), the diffuse nature of FR I sources, the brightness asymmetries of their jets decreasing with distance from the core, and often, the strong bends seen in FR I jets, imply that much slower flows exist on larger scales (e.g., O'Dea 1985; Feretti et al. 1999; Laing et al. 1999, and references therein).

Soon after their discovery it was noted that FR I sources tend to be associated with dynamically evolved cD or D type galaxies in clusters; in contrast, the hosts of FR II sources at similarly small redshift appear to avoid rich clusters (Longair & Seldner 1979; Seldner & Peebles 1978; Prestage & Peacock 1988; Owen & Laing 1991; Zirbel 1997), but more frequently have companion galaxies and/or isophotal distortions which signify recent galaxy mergers (e.g., Heckman et al. 1986; Hutchings 1987; Baum et al. 1992; Zirbel 1997). Furthermore, the hosts of FR I's are found to have an excess in optical size (relative to radio-quiet ellipticals of the same optical magnitude) that correlates with [FORMULA], while no such correlation is found for FR II's (Zirbel 1997). Hill & Lilly (1991) argued that the environment of FR II sources changes with cosmic epoch, in the sense that by moderately high redshifts ([FORMULA]), they begin to be found inside rich clusters; also see Zirbel (1997). Recent work (McLure & Dunlop 2000; Wold et al. 2000), however, indicates that there may be negligible cosmological evolution of the environment, and that powerful AGN do not really avoid clusters, even at small redshifts. Similarly, the well known statistical trend for the host galaxies of FR II sources to be about 0.5 magnitude fainter than those of FR I sources (Lilly & Prestage 1987; Prestage & Peacock 1988; Smith & Heckman 1989; Owen & Laing 1991) has recently been explained as a selection effect arising from a combination of the [FORMULA] dependence of the radio power at the FR I/FR II transition and the steepness of the radio luminosity function of elliptical galaxies (Scarpa & Urry 2000).

Additional differences between the optical properties of the host galaxies of FR I and FR II sources have been noted. Although FR II's exhibit roughly an order of magnitude stronger optical line emission than do FR I's of the same radio luminosity, the optical line emission seems to correlate with the host's optical magnitude only for FR I's (Baum et al. 1992; Zirbel & Baum 1995). The indicated internal origin of the gas in FR I's would be consistent with the recently inferred origin of dusty material detected in FR I sources, which appears to be generated either within the elliptical host itself, or acquired in close encounters with gas-rich galaxies (in contrast to an origin through violent mergers in the case of FR II's) (de Koff et al. 2000). Further, in contrast to FR II's, only a weak correlation of optical line emission with [FORMULA] is found for FR I's (Baum et al. 1995). Spectroscopic observations have indicated that the kinematics of the ionized gas in FR I's is turbulence dominated, while some ordered bulk rotation is present in the ionized gas associated with FR II hosts, and this rotation axis tends to be aligned with the radio axis (Baum et al. 1992). The emission line ratios of these "rotator"-type nebulae found in FR II sources are consistent with photoionization by the nuclear continuum, since the [O I] 6300, [N II] 6584 and [S II] 6717 forbidden lines are very weak relative to H[FORMULA]. Recent HST images of FR I nuclei reveal a deficiency of the thermal UV emission which is usually attributed to the nuclear accretion disk; this may account for the faintness of nuclear optical line emission (Chiaberge et al. 1999; also Zirbel & Baum 1995). Interestingly, Chiaberge et al. (2000) also found a similar situation for some of the FR II sources of modest radio luminosity.

Another notable difference between FR I and FR II hosts pertains to the amount of mid/far-IR (MFIR) emission: for samples matched in radio luminosity, FR II hosts typically have [FORMULA] times stronger MFIR emission, perhaps attributable to nuclear starbursts induced by galaxy mergers, which is also consistent with the higher rate of occurrence of optical distortion found for the FR II host galaxies (Heckman et al. 1994).

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000