Dependence of Faraday Effects on Radio Luminosity and Redshift

Abstract

Quantitative estimates are provided for the influence of luminosity selection effects present in a bright source sample on the observed correlation of the depolarisation rate Λ ½ with redshift z . It is shown that the observed correlation is partly intrinsic and partly due to luminosity effects. More specifically, we find that ~46% of the observed Λ ½ correlation with z can be attributed to strong luminosity selection effects in the sample. This shows that a significant fraction (~54%) of the observed correlation is cosmological in origin. It is concluded that there is a cosmological evolution of Λ ½, over and above that resulting from luminosity selection effects, which may be related to the epoch dependent properties of the ambient medium through which the radio source components propagate.