A VLA and MERLIN study of extragalactic radio sources with one-sided structure

Summary To investigate the nature of extragalactic radio sources with extended emission on only one side of the active nucleus, we have observed a sample of suspected candidates with both the VLA and MERLIN. In this paper, we present observations of 13 sources which were either suspected of being ‘one-sided’ or were known to have lobes of very different surface brightnesses. Many of the candidates were found to have emission on both sides of the nucleus. Using our observations as well as those available in the literature, we have compiled a sample of 4C quasars having one-sided radio structure. We have then compared some of their properties with a sample of 4C quasars which are two-sided, in order to determine whether their apparently onesided structure is due to relativistic beaming in sources inclined at small angles to the line-of-sight. We find that the median values of fc, the fraction of emission from the core used as a statistical measure of source orientation, and l, their projected linear sizes are consistent with the hypothesis that the one-sided quasars are seen at smaller angles to the line-of-sight than the two-sided ones. We also suggest that the ϕ-fc relation for quasars, where ϕ is the angle between the core polarization vector and the overall radio axis, is consistent with this model, and discuss how the lobe depolarization properties may be useful for further investigations of this interpretation. The velocities required to explain the apparently one-sided quasars in the relativistic beaming framework are in the range of ˜ 0.2-0.8 c. The scarcity of highluminosity, Fanaroff-Riley class II radio galaxies with one-sided structure is consistent with the suggestion that they are at larger angles to the line-of-sight than quasars. In this case, the core-dominated radio galaxies are possibly the lowluminosity, Fanaroff-Riley class I radio galaxies seen at small angles to the line-ofsight, manifesting themselves as BL Lac objects in the more extreme cases.