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We present the results of a Faraday rotation survey of 61 radio-bright QSOs conducted at the NRAO Very Large Array (VLA). The Galactic contribution to the Faraday rotation is estimated and subtracted to determine the extragalactic rotation measure (RRM) for each source. Eleven of these QSOs are known to exhibit damped Lyα absorption. The rate of incidence of significant Faraday rotation of these 11 sources is compared to the remaining 50 and is found to be higher at the 99. 8% confidence level. However, as this is based upon only two detections of Faraday rotation in the damped Lyα sample, the result is only tentative. If the two detections in the damped Lyα sample are due to the absorbing systems, then the inferred rotation measure induced by these systems is roughly 250 rad m^-2^, The two detections were for the two lowest redshift absorbers in the sample. We find that a rotation measure of 250 rad m^-2^ would have gone undetected for any other absorber in the damped Lyα sample due to the (1 + z) ^-2^ dilution of the observed RRM with redshift. Thus the data are consistent with, but do not prove, the hypothesis that Faraday rotation is a generic property of damped Lyα absorbers. We do not confirm the suggestion that the amplitude of RRMs increases with redshift. Rather, the data are consistent with no redshift evolution. We find that the uncertainty in the estimation of the Galactic rotation measure (GRM) is a more serious problem than previously realized for extragalactic Faraday rotation studies of QSO absorbers. A careful analysis of current methods for estimating GRM indicate that it can be determined to an accuracy of about 15-20 rad m^-2^. Previous studies underestimated this uncertainty by more than a factor of 2. Due to this uncertainty, rotation measures such as we suspect are associated with damped Lyα absorption systems can only be detected at redshifts less than z~1.
Oren et al. (Thu,) studied this question.