Faraday Rotation as a Diagnostic of Galactic Synchrotron Contamination in CMB Maps

We present a diagnostic test of possible synchrotron contamination in cosmic microwave background sky maps. The method involves a non-parametric measurement of cross-correlation between the Faraday rotation measure (RM) of extragalactic sources and the measured microwave signal at the same angular position. We argue that statistical properties of the observed distribution of rotation measures are consistent with a galactic origin, an argument reinforced by a direct measurement of cross-correlation between galactic synchrotron emission and RM values. We do not find any statistically compelling evidence for correlations between the RM values and the COBE DMR maps at any frequency. On the other hand, there is a statistically significant correlation with the preliminary WMAP individual frequency maps which remains significant in the Tegmark et al. Wiener-filtered map but not in the Internal Linear Combination map produced by the WMAP team.