Magnetic fields of active galactic nuclei and quasars: Redshift dependence

The observed reduction in the fraction of quasars with polarized radiation as the redshift increases is explained by an increase in the magnetic-field strength in accretion disks at high z. The emerging Faraday rotation causes the radiation to be depolarized. This mechanism allows the magnetic fields in accretion disks to be estimated at several hundred gauss. We give simple asymptotic formulas that describe the Faraday depolarization in optically thick accretion disks.