The rotation measure distribution of QSOs and of intervening clouds - Magnetic fields and column densities

Abstract

The redshift dependence of the extragalactic component of the rotation measure is analyzed for a set of 116 QSOs. The data and distribution of their residual rotation measures (RRM) as a function of emitting source redshift z(e) are discussed. Two possible classes of sources for the observed distribution of QSO RRM are considered in detail: the QSOs themselves and intervening clouds. Expressions for the functions of the theoretically predicted distribution and for its variance are developed for any observed z(e) for both of these populations. Best fit values for the characteristic rotation measure and its z dependence are determined for each of the two populations. Results considering each as the sole source of the RRM and considering them in combination are presented. The physical implications of the results are addressed and the physical properties of the likely candidates for the intervenors and their z-evolution are discussed in detail.