Galactic and intergalactic Faraday rotation

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 & Nelson. 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 ~ 9 Mpc, electron densities $$\sim\,{10}^{-4}\,\text{cm}^{-3}$$ and magnetic fields $$\sim\,0.1\,-\,1\,\mu\,\text{G}$$. The inferred filling-factor $$\sim\,2\,\times\,{10}^{-3}$$, implies $$\Omega \,\sim \,0.1$$.