The VLA and internal structures of jets

VLA images of bright, well-resolved FRI and FRII jets have shown that they exhibit a rich variety of internal knots, rings, loops and filaments, including some apparently helix-like structures. These internal details also provide input to models of jet physics, including:

• In FRI sources, as NGC315 shows quite dramatically (see Figure 5), jet collimation is not determined once and for all on parsec scales. The side-to-side brightness asymmetries of FRI jets also decrease with distance from the nucleus, and towards the outer edges of the jets from their axes (Laing 1996). The innermost regions of FRI jets also resemble FRII jets (well-collimated and dominated by magnetic field components parallel to the jet axis). The collimation and internal structure of these jets evidently change dramatically on just the scale where their sidedness characteristics also change.
• The detailed radio structure of M87's jet closely resembles that of the optical jet (Biretta 1996), and superluminal proper motions have been inferred for several features (Biretta et al. 1995).
• Semi-periodic strings of knots are seen in some FRII jets (e.g.,  Bridle et al. 1994). A variety of phenomena could produce this effect: nonlinear growth of Kelvin-Helmholtz instabilities, criss-crossing oblique shock patterns in confined jets, or regular perturbations of the flow velocity or direction. Higher resolution imaging and polarimetry (e.g.,  with the upgraded VLA) are needed to clarify the nature of this phenomenon, which could lead to (model-dependent) estimates of Mach number in the jets.
• In the jets of some FRII sources, limb-brightening and flat-topped emission profiles (Swain et al. 1996, 1998; Carilli et al. 1996, their Figure 5) imply diminished emissivity on the jet axis relative to the jet edges, in regions away from bright knots. It is not yet clear whether this effect is due to differential Doppler boosting in a relativistic jet with a slow boundary layer (as suggested below) or to enhanced in situ particle acceleration or magnetic field amplification in a boundary layer. In either case, our visualization of phenomena in FRII jets may be biased toward their outer (boundary) layers. Again, higher angular resolution and sensitivity (e.g., an upgraded VLA) are needed to explore this fully.

  

Figure 5: The recollimation of the NGC315 jets on 20 to 30-kpc scales is clear in this VLA 4885 MHz image at 2" resolution. The data have been rotated so that the axis of the large-scale jet is horizontal. The data combine syntheses in the B, C and D configurations (Cotton, W.D., Bridle, A.H., Laing, R.A., & Giovannini, G., in preparation.)

Alan Bridle
Tue Oct 13 15:58:42 EDT 1998