Absorption lines, Faraday rotation, and magnetic field estimates for QSO absorption-line clouds

We have estimated the extra galactic component of Faraday rotation for a sample of 37 QSOs for which there is good absorption line data, which we have also analyzed. Statistical evidence is presented which suggests that we have isolated a component of Faraday rotation which is occurring in the absorption clouds of some QSOs. Provided the ray paths for both spectral absorption and Faraday rotation are close, the combined optical and radio data enable us for the first time to estimate, or limit, the magnetic field strengths in intergalactic absorption clouds over a wide range of redshift. For 10 QSOs we have the first preliminary estimate of or limits to the magnetic field strengths of QSO absorption line systems, which range in redshift from 0.395 to 2.78. For seven of these the magnetic field strengths are less than a few tens of microgauss. In two other systems (PKS 0458-02 and 4C 24.6 1) where Za > Ze our straightforward estimate gives averaged field values of milligauss order. In these latter cases we propose an associated hot halo not thus far detected in absorption, whose presence would greatly reduce the derived magnetic strengths. There is also a possible tendency for "excess" extragalactic Faraday rotation to be associated with strong low-excitation absorption systems at one or two absorption-line redshifts, as distinct from multiple and high-excitation systems. This is consistent with the presence of intervening galaxy halos whose Faraday rotation we are detecting. It is concluded that most absorption line clouds have at least galaxy-like dimensions, i.e., ≳30 kpc. in order to produce the positive association between detectable rotation measure and strong optical absorption which we have found.