A Critical Look at Faraday Rotation Measures of Cluster Magnetic Fields

Strong magnetic fields (up to 40 μG) have been claimed for the ICM in galaxy clusters, especially those with strong cooling flows, based on studies of radio source rotation measures (RMs). The suggested evidence is of two types---studies of variations in RM across individual radio galaxies embedded in clusters, and statistical studies of background sources. We have re-examined this evidence, and find that no claims for such strong, cluster-wide fields can be supported against likely alternative explanations for the observations. We show that RM variations in embedded sources have a significant contribution from the medium local to the sources themselves, which would have to be removed before seeking evidence for cluster-pervading fields. We find that the statistical conclusions are based not on background sources, but on sources embedded in the clusters and others with unreliable RMs. When such inappropriate sources are eliminated from existing samples, only marginal evidence remains for cluster-wide fields from the RM data. Other indicators of cluster fields, such as radio halos, suggest field strength values at levels of μG or less. At such levels, the magnetic fields probably do not directly affect the dynamics of the intracluster medium, although they may still be important, e.g., for thermal conduction.