Lobe asymmetry and the expansion speeds of radio sources

Observations of the ageing of the synchrotron radiation spectrum in radio galaxies and quasars have led to estimates of the advance speed of the hotspots of only a few per cent of c for most radio galaxies but rising to 0.2c or more for the most powerful sources. Such large growth rates should result in measurable asymmetry of the observed lobe lengths, owing to the different light travel times. The original form of that test yielded upper bounds of 0.2c to 0.3c, but if we assume that the presence of a jet indicates the approaching lobe then a sharper version of the test becomes possible. This paper investigates the predicted statistical excess of jet-side length over counterjet-side length in three samples of very powerful extragalactic radio sources. In each case it is found that the excess indicates growth rates below 0.1c; although each estimate has only marginal statistical significance by itself, the agreement between the samples suggests that backflow plays at least as large a part as advance speed in determining the speeds obtained from synchrotron ageing observations of these sources.