Luminosity Selection Effects in the Observed Variation of Depolarisation Asymmetry Ratio with Redshift

The variation of the depolarisation asymmetry ratio (rdp) with redshift (z) for extended extragalactic radio sources has been examined taking into consideration the strong luminosity selection effects usually present in most well-studied flux density-limited samples. It is found that a greater fraction (~ 84%) of the observed rdp-z correlation is an artefact of the luminosity effects present in the sample. This result suggests that the cause of the strong correlation between observed Faraday effects and jet-sidedness (the Laing-Garrington effect) has no possible cosmological interpretation. The present result thus indicates that relativistic Doppler effects may be more fundamental in explaining the Laing-Garrington effect than the interaction of the radio source with an epoch-dependent asymmetric ambient medium. This result, however, is expected to apply more to quasars than to radio galaxies since the present sample consists largely of quasars (> 80%).