The incidence of Faraday rotation in a sample of radio-selected QSOs is combined with the incidence of foreground metal-line absorption to compute the probability for Faraday rotation in various types of metal- line absorbers. We divide the sample into subsets with and without damped Ly α absorption. The probability for Faraday rotation is significantly higher in the damped subset. Moreover, the probability is higher in the damped subset than in nondamped subsets selected on the basis of Mg II or C IV absorption. Owing to evidence linking damped systems to the progenitors of galactic disks and the Mg II systems to the progenitors of galactic halos, we conclude that magnetic fields were significantly higher in protogalactic disks than in proto-galactic halos. We estimate that the B fields in two damped Ly α systems with z ~ 2 are a few microgauss. The existence of microgauss magnetic fields in rotating galactic disks at these redshifts is extremely difficult to understand if magnetic field amplification is due to a global galactic dynamo. This is because there is not enough time available for amplification of an infinitesimal seed field of B ~ 10^-21^ G to B ~ 10^-6^ G by z ~ 2 in any standard Friedmann cosmology. The observations thus indicate either a more rapid amplification mechanism, possibly by a rapid local dynamo, or the presence of a finite primordial B field.
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Wolfe et al. (1992) studied this question.