In this paper we investigate the correlations between spectral index, jet side and extent of the radio lobes for a sample of nearby FRII radio galaxies. In an earlier paper we studied a sample of quasars and found that the high surface brightness regions had flatter spectra on the jet side (explicable as a result of Doppler beaming) whilst the extended regions had spectral asymmetries dependent on lobe length. Unified schemes predict that asymmetries resulting from beaming will be much smaller in narrow-line radio galaxies than in quasars, and we therefore chose to investigate, in a similar fashion, a sample of radio galaxies with detected jets. We find that spectral asymmetries in these objects are uncorrelated with jet sidedness at all brightness levels, but depend on relative lobe volume. Our results are not in conflict with unified schemes, but suggest that the differences between the two samples are primarily due to power or redshift, rather than to orientation. We also show directly that hotspot spectra steepen as a function of radio power or redshift. Whilst a shift in observed frequency owing to the redshift may account for some of the steepening, it cannot account for all of it, and a dependence on radio power is required.
Monthly Notices of the Royal Astronomical Society, 304, 271-280 (1999)
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