1. INTRODUCTION

Beam models were originally postulated to explain powerful, extended extragalactic radio sources (Rees 1971; Blandford & Rees 1974; Scheuer 1974) and gained much of their support from the observed ubiquity of jets in these objects. With one exception, M 87 (Biretta, these proceedings), none of the sources in which jets have been detected shows any direct evidence of motion on kiloparsec scales. Nevertheless, there are many indirect arguments, most notably the continuity from parsec scales (where motion is observed), which justify the conventional assumption that jets are fairly direct tracers of well-collimated, continuous, outflowing beams of material.

The present review does not attempt to cover the entire field in detail but, after outlining the basic systematics, focuses on attempts to derive the parameters of radio jets (especially their velocities) using simple physical diagnostics and indirect arguments about relativistic flow and sidedness. The recent and comprehensive discussions in Beams and Jets in Astrophysics (Hughes 1991) are recommended as an introduction to the whole field. The review by Bridle & Perley (1984) identified many of the systematic properties of extragalactic jets and remains essential reading.

Section 2 of the present study outlines the morphological classification of extended extragalactic radio sources and goes on to contrast the properties of strong and weak radio jets, emphasizing sidedness, magnetic field and collimation properties. Diagnostics of physical conditions are critically reviewed in Section 3: by contrast with the situation for stellar jets, all of the fundamental fluid parameters are poorly determined. The remainder of the review is concerned with a more detailed discussion of the debate on sidedness and relativistic flow on large scales (Section 4). Finally, Section 5 postulates a set of flow parameters which appear, at least to the present author, to be appropriate to jets in the various sorts of extragalactic radio source.

Throughout this review, it is assumed that the Hubble constant is H₀ = 100 kms^-1 Mpc^-1 and that the density parameter ₀ = 0. Monochromatic radio luminosities are denoted P_v, where v is the frequency in GHz. The discussion is restricted to radio sources in E and S0 galaxies with P_1.4 10¹⁹ WHz^-1, and ignores Seyfert and starburst galaxies.