Faraday rotation as a diagnostic of Galactic foreground contamination of cosmic microwave background maps

Abstract

We present a diagnostic test of possible Galactic contamination of cosmic microwave background (CMB) sky maps designed to provide an independent check on the methods used to compile these 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 the dust, free-free and synchrotron foreground maps and the RM values, with the strongest correlation being for dust and free-free. We do not find any statistically compelling evidence for correlations between the RM values and the COBE Differential Microwave Radiometer (DMR) maps at any frequency, so there is no evidence of residual contamination in these CMB maps. On the other hand, there is a statistically significant correlation of RM with the preliminary Wilkinson Microwave Anisotropy Probe (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.