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




The contribution from the residuals of the foreground can have a significant impact on the temperature maps of the cosmic microwave background (CMB). The most powerful residuals of the foreground are found in the Galactic plane. However, in this paper, we will focus on the possible foreground contamination from sources outside the Galactic plane in the CMB maps.

We will analyse the correlation between Faraday rotation maps and CMB temperature maps. A map of the Faraday rotation is dependent on the Galactic magnetic field, as well as the thermal electron density, and both may contribute to the measurement of the CMB temperature. We find that the standard deviation for the mean cross-correlation deviates from that of simulations at the 99.9 per cent level. Additionally, a comparison between the CMB temperature extrema and the extremal points of the Galactic Faraday depth is also performed, showing overlap for localized areas. Also, we find that the CMB cold spot is located at an area of strong negative cross-correlation, meaning that it may be explained by a Galactic origin.

Further, we investigate nearby supernova remnants in the Galaxy traced by the Galactic radio loops. These supernova remnants are located at high and low Galactic latitudes, and thus well outside the Galactic plane. We find some correlation between the Galactic Faraday depth and the CMB temperature at select radio loops. This indicates that the Galactic foregrounds may affect the measured CMB temperature at high Galactic latitudes.