Lowering Inferred Cluster Magnetic Field Strengths: The Radio Galaxy Contributions

We present a detailed examination of the relationship between the magneticfield structures and the variations in Faraday Rotation across PKS1246-410, aradio source in the Centaurus cluster of galaxies, using data from Taylor,Fabian and Allen. We find a significant relationship between the intrinsicposition angle of the polarization and the local amount of Faraday Rotation.The most plausible explanation is that most or all of the rotation is local tothe source. We suggest that the rotations local to cluster radio galaxies mayresult from either thermal material mixed with the radio plasma, or from thinskins of warm, ionized gas in pressure balance with the observed galaxy or hotcluster atmospheres. We find that the contribution of any unrelated clusterRotation Measure variations on scales of 2 - 10 arcsec are less than 25rad/m^2; the standard, although model dependent, derivation of cluster fieldswould then lead to an upper limit of approximately 0.4 microGauss on thesescales. Inspection of the distributions of Rotation Measure, polarisation angleand total intensity in 3C75, 3C465 and Cygnus A also shows source-relatedFaraday effects in some locations. Many effects can mask the signatures oflocally-dominated RMs, so the detection of even isolated correlations can beimportant, although difficult to quantify statistically. In order to use radiosources such as shown here to derive {\it cluster-wide} magnetic fields, as iscommonly done, one must first remove the local contributions; this is notpossible at present.Comment: 19 pages, including 3 color and 8 b/w figures, Accepted, Astrophysical Journa