Annu. Rev. Astron. Astrophys. 1997. 35: 607-36 Copyright © 1997 by Annual Reviews. All rights reserved

2.2. FR II Radio Galaxies and Quasars

Relativistic beaming models attribute the diversity in properties of extragalactic radio sources to orientation effects (e.g. Scheuer & Readhead 1979, Blandford & Königl 1979, Orr & Browne 1982, Barthel 1989). The cores of extended radio galaxies and quasars tend to be much weaker and presumably are not strongly relativistically beamed towards us. The systematic study of morphology and parsec-scale jet velocities offers the opportunity to test the beaming hypothesis by measuring the distribution of parsec-scale jet speeds. This requires selection of samples with minimal orientation bias, e.g. samples of lobe-dominated quasars from low-frequency surveys. So far, there is an intriguing trend that the observed speeds (in eight cases to date) are in the range v_app ~ 1-5 h^-1 c (Hough et al 1996a, Hough 1994, Hough & Readhead 1989, Vermeulen et al 1993, Porcas 1987, Zensus & Porcas 1987). The quasar 3C 263 is fascinating; this source shows a one-sided core-jet morphology similar to that of core-dominated sources like 3C 345, with evidence for mild acceleration and nonradial motion of jet components (Hough et al 1996a). So far, the relatively low superluminal speeds, together with these properties in 3C 263, tentatively support the unification of core- and lobe-dominated quasars.

Cygnus A (3C 405) is the closest luminous radio galaxy (see Carilli & Barthel 1996, Carilli & Harris 1996, and references therein). It is presumably oriented at a large angle (> 60°) to the line of sight and presents the best-studied case of a parsec-scale jet in a FR II radio galaxy (Carilli et al 1994, 1996, Bartel et al 1995, Krichbaum et al 1996, 1997). The parsec structure contains a knotty jet and a weak counterjet. The jet extends to about 300 h^-1 pc from the core (at 1.6 GHz), it is well collimated, and it contains subluminal component motion, measured at 5 GHz in the range 0.35-0.55 h^-1 c. There is no emission gap between jet and counterjet observed, in contrast to some inner-jet model predictions (Marscher 1996).

Close to the radio core of Cygnus A (Figure 3), the observed speed is even lower (0.1-0.2 h^-1 c), which indicates the presence of acceleration or pattern speeds (Krichbaum et al 1997). Oscillations of the jet ridge line and jet width may reflect jet instabilities (e.g. Kelvin-Helmholtz), which in turn would predict a pattern speed slower than the fluid speed (Hardee et al 1995). Assuming intrinsic symmetry, the frequency-dependent jet-to-counterjet ratio can be explained by partial obscuration of the counterjet, perhaps by a disc or torus (see Carilli & Barthel 1996). Overall, the properties of Cygnus A jet are compatible with the simple relativistic beaming model; the speed of the jet fluid at larger core-distances is 0.4 0.6 and the inclination is 35° 90° (Bartel et al 1995). However, given the slow apparent speed, the unification of Cygnus A type objects with blazars is difficult unless the intrinsic-jet Lorentz factor in this source is similar to that of quasars (Krichbaum et al 1997).

Figure 3. The inner jet and counterjet of Cygnus A at 22 GHz, at epoch 1994.17 (from Krichbaum et al 1997). The field of view is ~ 100 × 30 milliarcsec. The size of the circular beam is 0.7-mas size and the contour levels are -0.05, 0.05, 0.15, 0.3, 0.5, 1, 2, 5, 20, 50, and 90% of the peak flux density of 0.73 Jy/beam. The total flux density of the map is 1.65 ± 0.05 Jy.