The decelerating-jet picture of FRI sources associates their apparently parallel-field regions with the jet boundary layer, where we expect to find a strong velocity shear. The inter-knot emission from jets in FRII radio galaxies appears to be dominated by parallel-field configurations, so this emission may also be visualizing slower-moving material at the jet edges. If so, outflow velocities derived from the side-to-side brightness asymmetries of such jets may systematically under-estimate the velocities in the jet spines, which may stay hidden from us until they reach the hot spots. It will be important to learn whether the innermost regions of FRI jets, and of jets in FRII radio galaxies, generally appear edge-brightened, or at least flat-topped, when observed with good sensitivity and high transverse resolution. As mentioned earlier, there is evidence for a lack of emissivity near the jet axis in the large-scale jets of the FRII sources 3C353 (Swain et al. 1996, 1998) and Cygnus A (Carilli et al. 1996) but improved angular resolution and sensitivity (such as offered by the VLA Upgrade) will be needed to pursue these questions in the general population of FRII sources.
It will also be important to determine whether jets and counterjets in the same FRII source have similar transverse intensity and polarization profiles, and whether these profiles are systematically different for narrow-line radio galaxies, broad line radio galaxies, and quasars (which are expected to be at systematically different angles to our line of sight in orientation-based "unification schemes").
Although rapid deceleration of the FRI jets on kiloparsec scales may decollimate these jets, the jets also rapidly recollimate, as Figure 5 clearly shows. The hot, extended, component of the galactic (or circumgalactic) atmosphere required to produce the recollimation by thermal pressure alone has so far eluded detection at X-ray wavelengths in NGC315 (e.g., Birkinshaw & Worrall 1996). AXAF may tell us how serious this problem is for decelerating-jet models. Alternative explanations for the recollimation phenomenon might be found in the two-fluid (beam-wind) model of Sol et al. (1989), which seeks to explain it using J × B forces.
If decelerating-jet models of FRI sources pass further scrutiny, then jet kinematics inferred from VLA imaging may be used to constrain models of mass entrainment into relativistic jets traversing galactic atmospheres. The eventual goal should be to explain the major features of the collimation changes, brightness distributions, and polarimetry of these jets using a full dynamical model of the flows, rather than the simple kinematic modeling illustrated in Figure 6.
Alan Bridle
Tue Oct 13 15:58:42 EDT 1998