Annu. Rev. Astron. Astrophys. 1988. 36: 539-598
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2.2. Jets and Unified Models for Active Galactic Nuclei

Jets appear to be characteristic ingredients of all AGNs. Phenomenological scenarios in which Seyfert galaxies, radio galaxies, quasars, and blazars are interpreted as different manifestations of the same type of central power engine, with different powers and geometries, have become very popular. Some useful general references are the reviews by Blandford et al (1990), Antonucci (1993), Falcke (1996).

The key elements in these unification models are an unresolved accretion disk on scales << 1 pc and the twin opposite jets that are accelerated perpendicular to its plane: The jet/disk orientation with respect to the observer defines the visibility of the spectral components. In particular, jets are fully visible when they are perpendicular to the observer's line of sight, which is the case of extended radio galaxies and radio-loud quasars. Instead, jets are not clearly distinguishable when they are seen face-on, but correspondingly, their emission is relativistically Doppler boosted in frequency and luminosity, which is the case of blazars and compact radio galaxies. Weak AGNs would produce small jets, and the above classes would become Seyfert 2 (large angles to the line of sight), Seyfert 1 (smaller angles), and BL Lacs (face-on).

On scales on the order of parsecs, the accretion flow takes the form of a thick molecular torus that, depending on the orientation to the line of sight, can substantially obscure the inner engine. For lines of sight close to the torus equatorial plane, the central engine becomes completely invisible: The only visible components of AGNs are then the radio jets.

Along this scheme, and starting from the original classification of extended radio galaxies in strong jets (P178MHz > 5 × 1025 W Hz-1) and weak jets (P178MHz < 5 × 1025 W Hz-1) by Fanaroff & Riley (1974), two basic unification sequences have been proposed originally for radio-loud objects: (a) the sequence of radio galaxies with strong jets (FR II objects) -> quasars -> blazars, in which obscuration of the central engine decreases and beaming increases; and (b) the sequence of radio galaxies with weak jets (FR I objects) -> BL Lacs, in which again obscuration of the central engine decreases and beaming increases. Similarly, radio-quiet objects were unified in a third sequence of Seyfert 2 -> Seyfert 1 -> radio-quiet BL Lacs.

However, the original classification in two categories of radio-loud and radio-quiet AGNs is now overruled by more sensitive observations that indicate how a radio component is present in all cases, although with different powers. The more appropriate classes of radio-loud and radio-weak AGNs should now be used (Antonucci 1993). Therefore, Falcke & Biermann (1995), Falcke et al (1995) have analyzed the influence of the power of the central engine on morphologies in order to unify the above sequences. They found, in fact, a significant correlation between the ultraviolet-bump luminosity, taken as a measure of the disk (engine) luminosity, and the radio luminosity of the three sequences of AGNs. The radio-loud objects in fact do constitute a sequence going from FR I to FR II, quasars and blazars for increasing disk power, whereas radio-weak objects follow the same pattern at lower radio luminosity.

In particular, increasing the disk power produces stronger radio jets. Their emission is related mainly to internal relativistic electron acceleration. However, an important component in the initial propogation comes from the interaction of the collimated flows with the walls of the large-scale torus, leading to enhanced particle acceleration. The stronger the jet-torus interaction, the more prominent the jets. If the opening angle of the torus is assumed to be power dependent so that less powerful jets have narrower opening angles and suffer weaker jet-torus interaction, then this would explain the observational results that jets are systematically more prominent with decreasing radio power.

Finally, the difference between the radio-loud and radio-weak objects would be related to the fact that the first ones are produced by ellipticals and the latter by spirals. In this last case, the jets always would be weaker for the same central engine power and would suffer strong interaction with the external torus.

Given the simplicity of these arguments, Falcke et al (1995) suggested that observations support the idea that all AGNs have the same type of central engine, with their different morphologies produced by obscuration, beaming, and power. Incidentally, they also proved that the same correlation holds for stellar mass black holes with superluminal jets (Mirabel & Rodriguez 1994). A sketch of this classification is given in Figure 3.

Figure 3

Figure 3. A unified model of AGNs. The upper part of the drawing corresponds to high-power sources with the jet emerging from an open torus, the lower part to low-power sources with the jet emerging from a closed torus. Different morphologies are produced by the orientation of the observer with respect to the jet/obscuring torus. OVV, optically violent variables; RQ, radio-loud quasars; RG, radio galaxies; Sy, Seyfert galaxies.

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