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130 6. Summary and conclusions with the observed properties only when the field is wrapped around the radio lobes in a twodimensional geometry. In order to create RM bands with multiple reversals, more complex field geometries such as two-dimensional eddies are needed. 4. Magnetic field power spectrum: functional form and range of scales Recent observational work on galaxy clusters has led to claims that the intra-cluster magnetic field has a Kolmogorov-like power spectrum (i.e. a power law with slope q = 11/3). Kolmogorov turbulence would be expected over some inertial range if the turbulence is primarily hydrodynamic (Kolmogorov 1941) or for the MHD cascade investigated by Goldreich & Sridhar (1997). However, the assumptions on which the Kolmogorov turbulence relies (homogeneity, isotropic and lack of magnetization) are not satisfied in the intergalactic medium. Numerical simulations predict a wide range of spectral slopes (albeit with some preference for the Kolmogorov value; Dolag 2006), and a comparison between theory and observation therefore seems premature. None of the RM distributions studied in this thesis show fluctuations compatible with a Kolmogorov power spectrum over the full range of observed scales. Instead, they are consistent with flatter power-law power spectra (q≤3). A Kolmogorov form on scales below the resolution limit cannot be excluded, although this is not the case on linear scales as low as 60 pc in M 87. Power spectra with slopes flatter than 11/3 have also been found by other authors (Murgia et al. 2004; Govoni et al. 2006; Laing et al. 2008). Similar results come from the analysis of magnetic turbulence in the Galaxy. For example, Regis (2011) found a flatter index (q=2.7) by analysing the fluctuations in the Galactic synchrotron emission (see also Minter & Spangler 1996). It is worth noting that the power spectra for sources in this thesis appear to be steeper in richer environments. The slopes range from q≈2 in groups (0755+37, 0206+35) up to q≈3 in clusters (3C 353, M 87). The flatter slopes are found in sources showing anisotropic RM’s, but this might be a selection effect. The idea of that the index of the power spectrum is determined by the richness of the environment is probably oversimplified. In 3C 449, the power spectrum cannot be a single power law, but rather steepens on smaller scales. A broken power law gives a good representation of the data, as found for 3C 31 (Laing et al. 2008), but is not unique: a smoother function would fit just as well. The precise location of the break in the broken power law may not be physically significant, but a change in slope might occur at the typical field reversal scale if the field is produced by a fluctuation dynamo (Schekochihin & Cowley 2006; Eilek & Owen 2002). It is not clear what determines the minimum and the maximum scales of the magnetic field. Reconnection of field lines is expected to occur on much smaller scales than those sampled 130
6.7. Future prospects 131 and is therefore not relevant. Energy can be injected on large scales from the interactions between the radio source and the magnetized gas, gas motions (“sloshing”) within the group/cluster potential well (e.g. merger-related) and motion of the host galaxy. All of these provide energy input on roughly the outer scale estimated for 3C 449 and 0755+37 (the latter subject to confirmation by three-dimensional modelling). In principle, the origin of the maximum scale could be constrained by observations of the same type of radio source in different environments. 5. RM structure and its connection with the radio source morphology I pointed out a potential correlation between radio source morphology and RM anisotropy. RM bands have so far been unambiguously detected only in lobed radio galaxies. This is what expected if the process producing bands is related to some form of compression or draping. Radio galaxies with lobes are indeed expected to be more effective than tailed sources in compressing the surrounding gas. Production of RM bands by draped fields in tailed sources is not excluded, provided that there is relative motion between the source and the surrounding gas. This might occur in narrow-angle tail sources (where the host galaxy is moving rapidly through the ICM) or in wide-angle tails if there are sloshing motions of gas in the cluster potential well. 6.7 Future prospects The work described in this thesis raises a number of observational questions, including the following. 1. How common are anisotropic RM structures? Do they occur primarily in lobed radio galaxies with small axial ratios, consistent with jet-driven expansion into an unusually dense surrounding medium? Is their frequency qualitatively consistent with the two-dimensional draped-field picture? 2. Why do we see bands primarily in sources where the isotropic RM component has a flat power spectrum of low amplitude? Is this just because the bands can be obscured by largescale fluctuations, or is there a causal connection? 3. Are the RM bands suggested in tailed sources such as 3C 465 and Hydra A caused by a similar phenomenon (e.g. bulk flow of the IGM around the tails)? 4. Is an asymmetry between approaching and receding lobes seen in the banded RM component? If so, what does that imply about the field structure? 131
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Alma Mater Studiorum Università de
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To Hypatia from Alexandria in Egypt
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Contents Abstract i 1 Physics of ra
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4.6.3 The outer scale of the magnet
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Abstract The existence of diffuse m
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2. Two-dimensional Monte Carlo simu
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Chapter 1 Physics of radio galaxies
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1.2. Non-thermal emission mechanism
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1.2. Non-thermal emission mechanism
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1.3. Radio galaxies 7 FR II sources
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1.3. Radio galaxies 9 (a) (b) Figur
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1.3. Radio galaxies 11 pressure, th
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Chapter 2 The X-ray environment of
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2.2. The thermal component 15 Figur
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2.3. Radio source - environment int
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2.3. Radio source - environment int
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Chapter 3 Magnetic fields in the ho
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3.1. Introduction 23 Figure 3.1: Ex
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3.1. Introduction 25 to the relativ
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3.2. Faraday Rotation 27 The RM can
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3.2. Faraday Rotation 29 Figure 3.4
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3.3. Depolarization 31 3.3 Depolari
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3.5. The magnetic field profile 33
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3.6. Tangled magnetic field 35 rad
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3.7. The magnetic field power spect
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Chapter 4 Structure of the magneto-
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4.1. The radio source 3C 449: gener
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4.3. The Faraday rotation in 3C 449
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4.3. The Faraday rotation in 3C 449
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4.4. Depolarization 51 Figure 4.4:
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4.4. Depolarization 53 1.25 arcsec
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4.5. Two dimensional analysis 55 sm
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4.5. Two dimensional analysis 57 Fi
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4.6. Three-dimensional analysis 59
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4.7. Summary and comparison with ot
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Chapter 5 Ordered magnetic fields a
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5.1. The Sample 75 35 49 00 ROSAT P
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5.1. The Sample 77 5.1.1 0206+35 02
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Page 97 and 98: 5.3. Rotation measure images 81 IMA
Page 99 and 100: 5.3. Rotation measure images 83 cor
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Page 115 and 116: 5.8. Discussion 99 5.8 Discussion 5
Page 117 and 118: 5.8. Discussion 101 line−of−sig
Page 119 and 120: 5.8. Discussion 103 flat slopes. I
Page 121 and 122: 5.9. Conclusions and outstanding qu
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Page 125 and 126: Chapter 6 Faraday rotation in two e
Page 127 and 128: 6.2. Two-dimensional analysis: rota
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Page 137 and 138: 6.4. Preliminary conclusions 121 Ta
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Page 141 and 142: Summary and conclusions The goal of
Page 143 and 144: 6.5. Summary 127 responsible for th
Page 145: 6.6. General conclusions 129 radio
Page 149: Appendix 133
Page 152 and 153: 136 Data reduction and imaging of l
Page 160 and 161: 144 BIBLIOGRAPHY Bîrzan, L., Raffe
Page 162 and 163: 146 BIBLIOGRAPHY Ensslin, T.A. & Go
Page 164 and 165: 148 BIBLIOGRAPHY Jeltema, T.E. & Pr
Page 166 and 167: 150 BIBLIOGRAPHY Owen, F.N., 1989,
Page 168 and 169: 152 BIBLIOGRAPHY 152
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