How fast are the large-scale jets in quasars? Constraints on both Doppler beaming and intrinsic asymmetries

We analyse the jet-to-counterjet flux ratios measured from deep VLA images of 13 3CR quasars made by Bridle et al., in order to derive constraints on the degree of Doppler boosting (and hence the characteristic jet speeds on kiloparsec scales) and on how much intrinsic asymmetry between jets and counterjets is permitted. The main result is that the large jet-counterjet brightness ratios measured by Bridle et al. must be attributed almost entirely to the Doppler effect, requiring characteristic jet speeds ≥ 0.6c on kiloparsec scales. Modest intrinsic asymmetries are permitted, but do not reduce the characteristic speed significantly, while large asymmetries are ruled out by VLBI data. The data also require that orientations close to the plane of the sky are excluded (θmax < 75°), so at least some quasars masquerade as galaxies. for θmax in the range 45° to 60°, as suggested in certain unified schemes, the best-fitting jet speeds are in the range 0.6c to 0.7c. Analysis of jet-to-lobe ratios for a complete sample of 3CR quasars leads to an upper limit on jet speed of < 0.95c, demonstrating that there is at least some deceleration between parsec and kiloparsec scales.