Asymmetries in the jets of weak radio galaxies

We describe a study of the side-to-side asymmetries on kpc scales in the je ts of FR I radio galaxies selected from the B2 sample. The basic data are j et surface brightnesses and widths determined by fitting transverse profile s to Very Large Array (VLA) images at a range of distances from the core. D ifferences between the jets at a given distance from the nucleus are interp reted as effects of Doppler beaming on intrinsically symmetrical flows and are compared with the model derived for 3C31 by Laing & Bridle and with sim pler variants. The jet/counterjet brightness ratios where the main jet firs t brightens are correlated with core prominence, as expected for a relativi stic flow. From the distribution of brightness ratios, we infer that jets h ave a maximum velocity approximate to 0.9c where they first flare and brigh ten, but there is also evidence for additional slower material. Deceleratio n to subrelativistic speeds occurs on scales which increase with radio powe r. Jets in the majority of sources with luminosities < 10(24)WHz(-1) at 1,4 GHz become essentially symmetrical land therefore subrelativistic) within 2 kpc of the core. In more powerful sources, jets that flare within the fir st 2 kpc become symmetrical by 10 kpc, but a subset of the most luminous ob jects 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 f or a decelerating relativistic flow. Brightness and full width at half-maxi mum (FWHM) ratios are also anticorrelated, an effect which we interpret as a result of Doppler beaming for a flow in which the velocity decreases radi ally outwards from the jet axis. Jet deceleration by entrainment of externa l 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.