Gross morphology

A crucial morphological feature of extended extragalactic radio sources is that (see Chapter 2) there are actually two fundamentally distinct classes of object, in which the weaker, Fanaroff & Riley (FR) class I, sources are characterized by quasi-continuous luminous jets which often become distorted as they interact with the inter-galactic medium (Fig. 2.3), whereas the more powerful, FR II, sources have a simple, linear, double-lobed structure, with the brightest emission occurring in compact hotspots at the edge of each lobe (Fig. 2.8). A central challenge for any theoretical model of extragalactic radio source structure is therefore to reproduce this observed dichotomy, and to identify the factor, or factors, that determine which type of extended structure develops. Furthermore, we might hope that an improved theoretical understanding of the Fanar off & Riley classification would enable us to make improved, dynamical, estimates for the, observationally badly determined, physical parameters of jets in radio sources.

Although a complete model for radio source structure would probably have to involve variations in both the strength and direction of the central engine, and a non-uniform external medium, considerable insight into their gross morphology can be gained from axisymmetric simulations of steady jets in a constant ambient medium. This model, which we shall refer to as the basic model, has the additional virtue of being less computationally expensive than fully three-dimensional simulations, permitting a wider range of jet parameters to be investigated.