3.1. Images: Bends and Knots
Jets and related structures in extragalactic radio sources are the largest coherent physical structures in the universe, obtaining sizes of order 1 Mpc in some cases (DeYoung 1991). The remarkable collimation and stability of these jets remains somewhat of a mystery and a marvel (Icke 1991, Birkinshaw 1991).
A composite of images showing the Cygnus A jets at various scales is shown in Fig. 4. A jet towards the northwest lobe is clearly detected on both pc- and kpc-scales. The counterjet towards the southwest lobe is fainter, but also evident on both kpc-scales (Carilli et al. 1996), and on pc-scales (Krichbaum et al. 1996, Sorathia et al. 1996). The jet is well collimated and directed starting on scales 0.6 mas, and continuing to 60". Between 2 mas and 20 mas the jet position angle is 284° ± 2° . The kpc-scale jet also has a position angle of 284° ± 1° from 1" to 25" from the nucleus. The counterjet on kpc-scales has a position angle of 108° ± 2° , implying that the jet and counterjet are apparently misaligned (with respect to reflection symmetry) by 4° .
Figure 4. Contour images of the Cygnus A radio jet on various scales, reproduced from Sorathia et al. 1996. The top image shows kpc-scale radio source at 5 GHz with a resolution of 0.4". The contour levels are a geometric progression in 2, which implies a factor two rise in surface brightness every two contours. The first level is 2.25 mJy/beam. The middle image is of the inner kpc-scale jet of Cygnus A with the same contouring as above. The bottom image shows the pc-scale jet of Cygnus A at 3mas resolution (FWHM). The contouring scheme is the same as above, but the first contour level is 1 mJy/beam.
Out to 25" the jet is composed of a series of elongated knots, as can be seen in Fig. 5. The knots are all oriented at an oblique angle of about 7° to the jet axis. The knots are fairly regularly spaced by about 7". Beyond 25" the jet appears to `bifurcate', and gently curves towards the south, and then northward again, eventually becoming indistinguishable from filamentary structure in the lobe. Rudnick et al. (1994) and Carilli et al. (1989a) present images of the counterjet in Cygnus A revealing a dramatic bend by about 30° over a distance of about 30" before it enters the primary hotspot E in the southern lobe.
Figure 5. A high pass filtered image of the jet in Cygnus A made from an image at 5 GHz, 0.4" resolution, by subtracting a smoothed version of the same image from the full resolution image.
Gradual bends in radio jets could result from pressure gradients in the lobes, eg. due to large-scale turbulent eddies in the lobe back-flow (Icke 1991, Cox et al. 1991, Hardee and Norman 1990), or from jet instabilities (Hardee and Clarke 1992), or simply indicate the direction of ejection from the central engine. In the latter case the flow proceeds radially from the nucleus, and not along the observed jet structure. In Cygnus A there is marginal evidence that the magnetic field lines project parallel to the gradual curvature of the jet in the region between 25" to 45" from the nucleus. Such a field morphology is expected in the case when the jet flow is parallel to the observed curvature.
There have been many explanations proposed for knots in jets in powerful radio galaxies (Williams 1991), including internal shocks due to surface instabilities or changing surface pressure (Norman et al. 1982, Falle and Wilson 1985, Birkinshaw 1991), variations in the jet velocity (Rees 1978), bow shocks due to collisions with small, dense clouds (Blandford and Königl 1979), or oscillations between magnetic and thermal pressure dominance in an axisymmetric jet with a `pinching' toroidal magnetic field (Chan and Henriksen 1980). Two characteristic of the knots in Cygnus A which any model must address are that the knots are clearly not axisymmetric, and that they have a fairly regular pattern in terms of length, separation, and pitch angle.
One model which predicts a regular, non-axisymmetric pattern of knots in a jet has been presented by Königl and Choudhuri (1985). They calculate the expected surface brightness distribution of a pressure confined, magnetized jet which has relaxed to a `force-free' magnetic field configuration, i.e., no Lorentz force on the plasma. Their jet has a series of oblique knots separated by about five jet radii. They suggest that the energy supply for the synchrotron emitting plasma in jets is that dissipated in current sheets in field reconnection regions during the relaxation of the fields to the force-free configuration.
Hardee and Clarke (1992) present a strictly hydrodynamic 3D simulation of a pressure matched, high Mach, light jet, driven with small amplitude precession at the origin (to break axisymmetry). Their jet develops twisted filamentary surface structures due to higher order (`fluting' mode) Kelvin-Helmholtz surface instabilities, and an elliptical distortion in the jet cross-section, culminating in a helical bifurcation of the flow. They conclude that the edge-brightened, twisted appearance of the Cygnus A jet at 30" from the nucleus is reasonably modeled by this coupled helical-elliptical mode.
The results of Hardee and Clarke (1992) have been generalized to include magnetic fields by Hardee and Clarke (1995). Using a a 3D magnetohydrodynamical (MHD) simulation of a dense, supermagnetosonic jet, they find that the jet can maintain both a helical twist and an elliptical distortion of large amplitude over a distance 60 jet radii without disrupting. The predicted jet radio morphology includes a series of oblique filaments crossing the jet at regular intervals, much like that seen for the Cygnus A jet. A detailed comparison between this simulation and the observed structures of the Cygnus A jet by Hardee (1996) yields: Mj 10 and 2 j 6, where j is the jet Lorentz factor, j is the density ratio between the jet and the radio lobe, and Mj is the jet magneto-sonic Mach number.