Pacman (II): Application and Statistical Characterisation of Improved RM Maps

We proposed a new method – Pacman – to calculate Faraday rotation measure (RM) maps from multi-frequency polarisation angle data (Dolag et al.) in order to avoid the so-called nπambiguity. Here, we apply our Pacman algorithm to two polarisation data sets of extended radio sources in the Abell 2255 and the Hydra A cluster, and compare the RM maps obtained using Pacman to RM maps obtained employing already existing methods. Thereby, we provide a new high quality RM map of the Hydra north lobe which is in a good agreement with the existing one but find significant differences in the case of the south lobe of Hydra A. We demonstrate the reliability and the robustness of Pacman. In order to study the influence of map making artefacts, which are imprinted by wrong solutions to the nπ-ambiguities, and of the error treatment of the data, we calculated and compared magnetic field power spectra from various RM maps. The power spectra were derived using the method recently proposed by Enßlin & Vogt (2003). We demonstrate the sensitivity of statistical analysis to artefacts and noise in the RM maps and thus, we demonstrate the importance of an unambiguous determination of RM maps and an understanding of the nature of the noise in the data. We introduce and perform statistical tests to estimate the quality of the derived RM maps, which demonstrate the quality improvements due to Pacman .