The rotation measures of high luminosity sources as seen from the NVSS

We re-analyse the subset of the Faraday rotation measures data from the NRAO VLA Sky Survey catalogue for which redshift and spectral index information is available, in order to better elucidate the relations between these observables. We split this subset in two based on their radio luminosity, and find that higher power sources have a systematically higher residual rotation measure, once the regular field of the Milky Way is subtracted. This rotation measure stands well above the variances due to the turbulent field of our Galaxy and measurement errors, contrarily to low power sources. The effect is more pronounced as the energy threshold becomes more restrictive. If the two sets are merged one observes an apparent evolution of rotation measure with redshift, but our analysis shows that this can be interpreted as an artifact of the different intrinsic properties of brighter sources that are typically observed at larger distances.