Expansion speeds in extended extragalactic double radio sources from angular structure

It is shown that the distribution of the expansion speed v of the components of extragalactic double radio sources can be analytically derived from that of fractional difference x between the arms of the double. Assuming symmetric evolution of the components at constant speed v, the distributions p(v) and g(x) are related by p(v) = -v.g'(v) in which v is measured in units of c. p(v) is derived from g(x) for three subsamples of the 166 3CR sources and implies that about half of the double sources have between 0.1c & 0.4c, which is appreciably higher than Mackay's (1973) upper limit of approx. 0.08c derived from the width of flux density ratio distribution.