Investigations of Faraday Rotation Maps of Extended Radio Sources ...

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Abstract In this work, statistical analyses and tests of Faraday rotation measure (RM) maps are developed in order to determine the properties of the magnetic fields which are associated with the plasma in clusters of galaxies. Whenever linearly polarised radio emission passes through a magnetised medium, the plane of polarisation is rotated – an effect called Faraday rotation effect. It is sometimes debated which magnetised medium is mainly responsible for this Faraday rotation. Does the magnetised medium close to the source of the polarised radio emission dominate or does the Faraday rotation mainly take place in the magnetised medium which is located between the source and the observer, the intra-cluster gas? After investigations which provide confidence that the Faraday rotation is mainly due to the magnetised medium which is external to the source, the question arises how one can analyse the observed Faraday rotation maps to gain insight into cluster magnetic field properties. Therefore, a statistical approach to analyse the RM maps in terms of autocorrelation function and power spectrum is developed and applied to the data. Realising that map making artefacts and noise in the data have a noticeable effect on this analysis, especially for measuring power spectra, a new RM map making algorithm called Pacman is introduced. Pacman provides high quality RM maps which allow precise estimates of power spectra. In order to use the full potential of these maps, an accurate power spectrum estimator based on Bayesian maximum likelihood methods is developed and successfully applied to characterise cluster magnetic field properties such as magnetic field strength, magnetic field autocorrelation length and the shape of the magnetic power spectrum. In this work, the origin of the high Faraday rotation measures found for polarised radio galaxies in clusters is investigated. The two most likely sites of Faraday rotation are within magnetic fields local to the radio source, or within magnetic fields located in the foreground intra-cluster medium (ICM). In order to distinguish these two scenarios in the data, a gradient alignment statistic aimed to find correlations between the intrinsic polarisation angle ϕ 0 and RM maps is introduced. Such correlations were proposed to result in the case of source local RM screens. The statistic is applied to a number of extended radio sources in galaxy clusters (PKS 1246-410, Cygnus A, Hydra A, and 3C465). In no case, a significant large-scale alignment of ϕ 0 and RM maps is detected. However, a significant small-scale co-alignment is found in all cases. This co-alignment can be fully identified with map making artefacts through a suitable statistical test. At present, there is no existing evidence for Faraday rotation local to radio lobes. Given the existing independent pieces of evidence, it is concluded that the observed Faraday screens are produced by intra-cluster magnetic fields. Having realised this, a statistical analysis of RM maps is developed. The autoxi
Page 1 and 2: Investigations of Faraday Rotation
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Page 6 and 7: vi CONTENTS 3.4.2 Real Space Analys
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46 CHAPTER 3. MEASURING MAGNETIC FI
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72 CHAPTER 4. PACMAN - A NEW RM MAP
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100 CHAPTER 5. A BAYESIAN VIEW ON F
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118 CONCLUSIONS tually statisticall
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120 BIBLIOGRAPHY Conway, R. G. & St
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122 BIBLIOGRAPHY Ikebe, Y., Makishi
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124 BIBLIOGRAPHY Simard-Normandin,
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126 ACKNOWLEDGEMENTS in his long em
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Publications Journals 1. A Bayesian
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