Kinematics of kiloparsec-scale jets

It has long been known that kiloparsec-scale jets in radio galaxies can be divided into two flavours: strong (found in powerful sources, narrow and terminating in compact hot-spots) and weak (found in low-luminosity sources, flaring, unable to form hot-spots and terminating in diffuse lobes or tails). Both flavours are initially relativistic, but weak jets decelerate to sub-relativistic, transonic speeds by entraining external gas while strong jets remain relativistic and supersonic until they terminate. Much is now known about the kinematics of weak-flavour jets, which can be modelled as intrinsically symmetrical, decelerating relativistic flows, and we summarize the results of our work in this area. Strong-flavour jets are relatively faint and narrow, so it has hitherto proved difficult to obtain the necessary deep, transverse-resolved images in total intensity and linear polarization. The spectacular jets in the radio galaxy NGC 6251 appear to represent a transition case between weak and strong flavours: the jets show no clear evidence for deceleration, but are relatively wide. VLA observations hint at transverse velocity structure with a very fast (Lorentz factor >5) spine surrounded by a slower shear layer. New observations with the upgraded VLA should be able to test this picture.