Galactic and intergalactic Faraday rotation

Abstract

A model of the systematic component of the magnetic field of the Galaxy is fitted to a sample of 459 extragalactic rotation measures (RM), and the results are found to be consistent with a previous analysis of pulsar RMs by Thomson and Nelson (1980). The model is then used to reduce the effect of galactic Faraday rotation on the RMs of 134 QSOs, and the results used to investigate the existence of a Faraday-active intergalactic medium. Three models are considered in order to explain the redshift dependence of the RM variance. Although none of these models can be excluded, a significant fraction of the observed Faraday rotation may take place in extended cluster/supercluster haloes with dimensions about 9 Mpc, electron densities about 0.0001/cu cm and magnetic fields about 0.1-1 microgauss. The inferred filling-factor, about 0.002, implies omega of about 0.1.