Kinematics and dynamics of relativistic jets on large and small scales

Modelling of deep VLA images of the jets in FR I radio galaxies has allowed us to derive their three-dimensional distributions of velocity, emissivity and magnetic-field structure on kiloparsec scales. By combining our models of jet kinematics with measurements of the external pressure and density derived from Chandra observations, we can also determine the jet dynamics via a conservation-law approach. The result is a detailed and quantitative picture of jet deceleration. We discuss the potential application of these techniques on VLBI scales. Our fundamental assumption is that the jets are intrinsically symmetrical, axisymmetric, relativistic flows. Although this is likely to be a good approximation on average, the effects of non-stationarity in the flow (e.g. shocks/knots) may limit its applicability on pc scales. We also stress the need for both the main and counter-jet to be detected with good transverse resolution in linear polarization as well as total intensity. Two foreground effects (free-free absorption and Faraday rotation) must also be corrected. We comment on the implications of observed VLBI polarization and Faraday rotation for the field structure (and, by implication, collimation) of pc-scale jets. Where our VLA observations start to resolve the jets, about 1 kpc from the nucleus, they are travelling at about 0.8c. We briefly discuss the deceleration of jets from a much faster initial speed.