QSO absorption lines and rotation measures

Abstract

The origin of the extragalactic component of QSO Faraday rotation measure (RRM) is reconsidered in light of recent advances in the knowledge of QSO absorption and rotation measure. Kronberg & Perry suggested that there is a correlation between strong metal-line absorption and high rotation measure. Following this suggestion Welter, Perry & Kronberg analyzed the redshift dependence of the RRMs of 116 QSOs and concluded that the most likely origin of the observed RRM was extended galactic haloes or intragroup/cluster gas. This enabled them to set limits on magnetic fields in QSOs, intervening galaxies and clusters, and possibly even in the early Universe. Because of the importance of these results we have re-examined this question. Using the latest data on the clouds of the Ly-α forest, it is shown that their line-of-sight averaged magnetic fields would have to be more than an order of magnitude above the equipartition value if they were the origin of the observed QSO RRMs: we therefore rule them out. Turning to metal-line systems, the sample of QSOs for which both accurate RRMs and high-resolution absorption-line data are available is expanded and revised as a result of new observations. Within the restriction of smallnumber statistics, we confirm that RRM probably arises within the metal-line systems and not within the QSOs. We discuss several individual observations. Additionally, our results support the conclusions of O'Dea severely restricting the properties of the interstellar medium within QSOs.