1999MNRAS.306..513L

We describe a study of the side-to-side asymmetries on kpc scales in the jets of FR I radio galaxies selected from the B2 sample. The basic data are jet surface brightnesses and widths determined by fitting transverse profiles to Very Large Array (VLA) images at a range of distances from the core. Differences between the jets at a given distance from the nucleus are interpreted as effects of Doppler beaming on intrinsically symmetrical flows and are compared with the model derived for 3C31 by Laing & Bridle and with simpler variants. The jet/counterjet brightness ratios where the main jet first brightens are correlated with core prominence, as expected for a relativistic flow. From the distribution of brightness ratios, we infer that jets have a maximum velocity ~0.9c where they first flare and brighten, but there is also evidence for additional slower material. Deceleration to subrelativistic speeds occurs on scales which increase with radio power. Jets in the majority of sources with luminosities <10²⁴ W.Hz^–1 at 1.4 GHz become essentially symmetrical (and therefore subrelativistic) within 2 kpc of the core. In more powerful sources, jets that flare within the first 2 kpc become symmetrical by 10 kpc, but a subset of the most luminous objects has jets which remain asymmetrical to larger distances. The point at which the brighter jet flares appears to correspond to a sudden increase in rest-frame emissivity, but the ratio of distances to the flaring point in main and counterjets is anticorrelated with brightness ratio, as expected for a decelerating relativistic flow. Brightness and full width at half-maximum (FWHM) ratios are also anticorrelated, an effect which we interpret as a result of Doppler beaming for a flow in which the velocity decreases radially outwards from the jet axis. Jet deceleration by entrainment of external material provides a natural explanation for these velocity gradients. The jet energy flux is roughly consistent with energy supply to the lobes over a source lifetime estimated from spectral index measurements. Our results are qualitatively consistent with unified models of FR I radio galaxies and BL Lac objects, but require some modifications to the standard picture.