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2.1. Properties of extragalactic jets

The fact that synchrotron emission does not provide spectral lines has meant that even the simplest facts about the properties of jets, such as their velocities and compositions, have remained controversial. While magnetic field structures can be directly probed through polarization measurements, estimates of magnetic field strengths can usually only be made through the assumption of equipartition between the energies in the radiating relativistic particles and the magnetic field energy density; this assumption has the advantage of minimizing the total energy that needs to be supplied to the radio source.

Evidence for relativistic bulk motion in the powerful FR II sources has been generally accepted for quite a while, mainly because of the detection of apparent superluminal expansions of pc-scale knots through Very Long Baseline Interferometry (VLBI), along with the detection of fast motions on substantially larger spatial scales in a few sources. A source of radiation moving at moving with velocity beta = v / c at an angle, theta to the line-of-sight will have an apparent transverse velocity, betaapp = (beta sintheta) / (1 - beta costheta), which can exceed 1 if beta approx 1 and theta is small. Very fast motions, implying bulk Lorentz factors, 2 < Gamma = (1 - beta2)-1/2 < 10, also naturally explain the substantial asymmetry in the radio emissivities of extended jets in FR II sources; the jet with a component towards us would be Doppler boosted, while the (usually unobserved) counter-jet would be Doppler dimmed. For a jet of intrinsic spectral index alpha [defined so that Sem(nu') propto (nu')-alpha] the observed flux would be (e.g., Scheuer & Readhead 1979), Sobs(nu) = Sem(nu) D(2 + alpha), where the Doppler factor, D = [Gamma(1 - beta costheta)]-1], and the observed frequency is related to the emitted one by nu = nu'D; for small theta, this implies large enhancements. For a single source, the exponent in the above equation becomes (3 + alpha), and the boosting effect is even more pronounced. Apparently abrupt changes of directions of jets on the VLBI scales are also naturally explained in terms of small intrinsic direction changes that appear magnified to distant observers by special relativistic effects. Since the fluxes from the opposite hot-spots and lobes are usually very similar in FR II sources, it is widely accepted that the bulk motions of those extended regions are non-relativistic, with typical head advance speeds of ~ 0.03c (e.g., Scheuer 1995).

Recent observations have found that many FR I jets also provide evidence of relativistic velocities on VLBI scales (e.g., Giovannini et al. 1994, Biretta et al. 1999, Xu et al. 2000). Doppler boosting can not only explain that powerful FR II jets appear one-sided, but also that weaker FR I jets exhibit large brightness asymmetry only near their origins, and typically have short, one-sided basal regions (e.g., Laing et al. 1999; Kharb & Shastri 2001). The evidence that FR II jets retain relativistic bulk velocities out to 100's of kpc is quite convincing, with the brighter large scale jet always seen on the same side as the nuclear jet and towards the less depolarized radio lobe (e.g., Garrington et al. 1988, Bridle 1996, Gopal-Krishna & Wiita 2000a), so that it is inferred to be approaching us. However, for FR I jets, their diffuse morphologies, the fact that the brightness asymmetry of the jets decreases with increasing distance from the core, and substantial large bends frequently seen in FR I jets at multi-kpc scales imply that much slower flows (vbulk < 0.1c) exist on larger scales (O'Dea 1985, Feretti et al. 1999, Laing et al. 1999).

It is universally accepted that most, if not all, of the synchrotron emission arises from relativistic electrons. But the nature of the main positively charged component of the jet plasma remains contentious. Until recently, the great majority of workers have assumed that protons provided that neutralizing matter, and that their much greater masses implied that they required a lot of extra energy to accelerate but that they provided very little radio emission. However, the possibility that the jet was really composed of an e+-e- plasma has been suggested for a long time (e.g., Kundt & Gopal-Krishna 1980). Application of total energy and synchrotron radiation constraints led Celotti & Fabian (1993) to conclude that FR II jets were made of e--p plasma, since they argued that e--e+ plasma of the required density would yield too much annihilation radiation. However, Reynolds et al. (1996b) used similar energetic and radiation constraints to conclude that the jet in the FR I source M87 was likely to be made of e--e+ plasma. A similar argument favors an electron-positron jet in the Optically Violently Variable Quasar 3C 279 (Hirotani et al. 1999). If all of these arguments are taken at face value, one might infer that the main difference between FR I and FR II sources lies in the composition of the jet plasma, and this would imply the existence of a fundamental difference between their central engines. But it is worth noting additional evidence for the presence of pair plasma jets, even in FR II sources, comes from the interpretation of the radio power-linear-size (P-D) diagram in terms of a model for quasi-self-similar growth of double radio sources (Kaiser et al. 1997).

Although there are exceptions to this rule, it is well established that, in general, 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). Careful studies of FR I jets have led to the conclusion that the asymmetries in apparent emission from the two jets, and their detailed magnetic field patterns, are best explained if the jets in these sources consist of a narrow ``spine'' of relativistic flow with a predominantly transverse magnetic field, surrounded by a slower moving ``sheath'', probably contaminated by entrained material (a shear layer) where the magnetic field is stretched into a predominantly longitudinal configuration (e.g, Laing et al. 1999).

2.2. The Fanaroff-Riley Dichotomy

There are many other observed differences between FR I and FR II radio sources and the galaxies that host them (e.g., Baum et al. 1995, Zirbel 1997, Gopal-Krishna & Wiita 2000b). All these observations have led to the development of two general classes of explanations for the Fanaroff-Riley dichotomy.

Intrinsic explanations involve a fundamental difference in the central engine or jet properties between these two classes, while extrinsic explanations claim that the differences arise through interactions of the jets with the media through which they propagate. Among the intrinsic explanations are (see Gopal-Krishna & Wiita 2000b for details and many more references): the difference in jet composition mentioned above; a difference in the central engine, such as having a more rapidly spinning black hole yield FR II jets (e.g. Wilson & Colbert 1995, Meier 1999); a difference in the accretion process, where advection dominated flows might yield FR I jets, while more luminous ``standard'' accretion disks might produce FR II jets (e.g. Reynolds et al. 1996a).

The various extrinsic explanations assume that the jets differ in little except total power or thrust. In these scenarios, deceleration of the jet, through growth of instabilities and/or entrainment of external plasma, converts weaker jets into FR I morphologies, while stronger jets, which remain supersonic and/or relativistic to great distances, produce FR II structures (e.g., Bicknell 1984, 1995, Komissarov 1990). Recently Gopal-Krishna & Wiita (2000b) have stressed that the existence of a small number of sources with distinctly FR I morphologies on one side of the core and FR II morphologies on the other side, can play an extremely important role in distinguishing between these putative explanations for the FR dichotomy. They have found six good examples of these HYbrid MOrphology Radio Sources, or HYMORS, and have argued that while they are expected to be rare if an extrinsic mechanism dominates, HYMORS are unlikely to be found at all if an intrinsic mechanism were to be important.

As more measurements were made of the galaxies that host radio jet sources, it became clear that the simple radio source power criterion found by Fanaroff & Riley was not really appropriate. Rather, as the luminosity of the host galaxy, Lopt grows, so does the radio power, PR*, required to produce an edge-brightened, FR II, morphology. This was quantified by Ledlow & Owen (1996), whose extensive data compilations showed that PR* propto Lopt1.7. Bicknell (1995) demonstrated that this relation could be roughly reproduced within an extrinsic model for the FR dichotomy; in this picture, the weaker jets would slow until Gamma appeq 2 and then suffer the growth of instabilities that lead to FR I type structures. It has now been shown that a variant of this scenario, using a somewhat different jet propagation model (Gopal-Krishna et al. 1989), and where the trigger that yields FR I structure is now the slowing of the advance speed of the jet to subsonic with respect to the external ambient gas (Gopal-Krishna et al. 1996), can produce an even better fit to the observed PR* - Lopt relation (Gopal-Krishna & Wiita 2001). While both the magnetic-switch model (Meier 1999) and the gravitational slingshot model (Valtonen & Heinämäki 1999) can also yield rough agreements with the radio-optical correlation, given the additional evidence from the existence of HYMORS, it is clear that extrinsic explanations for the FR dichotomy are more likely to be correct.

2.3. Implications of Relativistic Motions

As mentioned in Section 2.1, the detection of apparently superluminal transverse motions provides extremely strong evidence for relativistic motions in jets, and these large Gamma values also provide an excellent explanation of the preponderance of asymmetric jet luminosities in double radio sources with quite similar lobe powers. The orientation at which we view these relativistic jets can also provide an understanding of several other key features of radio-loud AGN.

It is now widely accepted that AGNs with strong radio jets will typically be classified as radio galaxies if the orientation of the jet to our line-of-sight to the source is greater than a critical value, thetacrit appeq 40°, while the same source will be called a quasar if theta < thetacrit (e.g., Barthel 1989; Urry & Padovani 1995). These unified models for radio-loud AGN assume that the jets are launched parallel to the rotation axis of a supermassive black hole (SMBH) and perpendicular to the accretion flow feeding the SMBH. Unified models also require the presence of a thick dusty torus outside the accretion flow (on the scale of several parsecs) that can absorb enough soft X-ray, UV and optical radiation to hide both the direct core continuum emission and the broad emission line region if viewed from angles above thetacrit.

In addition, if the jet is very close to our line of sight (theta appeq Gamma-1) then very substantial special relativistic effects would strongly enhance the observed fluctuations and polarization. Under these circumstances the source might be classified as an Optically Violently Variable quasar or other type of blazar. Extremely convincing explanations for the variations on the timescales of months of several quasars in terms of shock-in-jet models have been available for quite some time (e.g., Marscher & Gear 1985, Hughes, Aller & Aller 1991), and very rapid variability and polarization swings can be understood if the shocks are travelling down a slightly bent jet (e.g., Gopal-Krishna & Wiita 1992) or if there is strong turbulence in the vicinity of the shock (e.g., Marscher & Travis 1991). Recent measurements indicate that much of the fastest (intraday) radio variability (see Wagner & Witzel 1995 and Wiita 1996 for reviews) is probably due to interstellar scintillation (ISS; Rickett 1990); two such sources are PKS 0405-385 (Kedziora-Chudczer et al. 1997) and J1819+3845 (Dennett-Thorpe & de Bruyn 2000). Nonetheless, extremely rapid intrinsic variations do appear to be required for some sources. One example is the BL Lac object 0716+714 where there appears to be a correlation between variations in the radio and optical bands (Wagner et al. 1996). Another case is the gravitational lens system B0218+357 (Biggs et al. 2001); here correlated variations between the two images over a few days are seen to be nicely separated by the 10.5 day time lag due to lensing, and cannot be explained in terms of ISS or gravitational microlensing (Gopal-Krishna & Subramanian 1991).

Turning to the weaker radio sources, there is now abundant evidence that FR I radio galaxies are the parent population for BL Lacertae objects, in the sense that a typical FR I source, if viewed at small theta, would show the properties of a blazar, and that the relative numbers of these classes are nicely understood if this unification holds (e.g., Urry & Padovani 1995). The synchrotron self-Compton mechanism can neatly explain the overall spectral energy distribution of blazars (e.g., Sambruna et al. 1996), but this only works if Gamma > 5, for otherwise X-rays would be over produced. Intrinsic variability in many blazars implies small linear sizes; these translate into brightness temperatures that substantially exceed the inverse Compton limit of ~ 1012 K for incoherent synchrotron sources unless similarly high values of Gamma are invoked.

As the sensitivity and dynamic range of radio observations have continued to improve it has become clear that many so-called radio quiet AGN are actually radio weak, but not radio silent, and that there is a considerable population of radio-intermediate sources (e.g., White et al. 2000). Quite a few Seyfert galaxies are now known to possess radio jets, though they tend not to propagate very far, probably because of more rapid disruption by the interstellar medium of their spiral hosts (e.g., Pedlar et al. 1993) Again, a unified scheme seems to work very well, with the Seyfert 1 galaxies viewed at theta < thetacrit so the broad line region and soft X-rays can be seen directly, while the Seyfert 2 galaxies are viewed at theta > thetacrit and broad lines can only be detected in polarized reflected light (cf. Antonucci 1993).

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