Faraday rotation from Abell clusters of galaxies

Observations performed with the Very Large Array (VLA) indicate that the distribution of rotation measures of sources seen behind Abell clusters of galaxies is not statistically different from the distribution of sources not behind clusters. A conservative upper limit of 55 rad/sq m is set for the amount of excess rotation which can be attributed to clusters of galaxies. This limit is influenced by the sources observed rather than by properties of the clusters. Assuming that the magnetic field of the cluster is not tangled, the conservative upper limit for excess Faraday rotation implies that the magnetic field strength is less than 6.8 x 10 to the -8th G, which is lower than limits set from either energy equipartition arguments from cluster halos or from inverse Compton limits from X-ray observations. The magnetic field is tangled on size scales of order a few tens of kiloparsecs, and the size scale for tangling is from 460 to 4 kpc, assuming a Hubble parameter of 75 km/s/Mpc. 24 refs.