Detection of excess rotation measure due to intracluster magnetic fields in clusters of galaxies

The Faraday rotation measures of a sample of extragalactic radio sources projected within a third of an Abell radius of a galaxy cluster were compared with those of sources located further from cluster centers. The result strongly indicates that the distribution of the residual rotation measure (RRM) in the former population is broadened, at a confidence level exceeding 99%. The broadening is detectable out to 1h^-1^₅0_ Mpc. Our best estimate of the excess Faraday rotation measure varies from 100+/-36 rad m^-2^ in the central sixth of an Abell radius to 36+/-15 rad m^-2^ further out. The combination of these results with electron densities determined from X-ray data for some of the clusters suggests that magnetic field strengths in cluster gas are of order 1 microgauss.