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128 6. Summary and conclusions In M 87, a power spectrum with slope q≃3.0 fits well everywhere. The minimum scale ranges from 0.06 up to 0.25 kpc. An approximate lower limit to the outer scale is 2.7 kpc. 6.6 General conclusions 1. Origin of the bulk of the Faraday effect observed across radio galaxies It is possible to establish that most of the RM is due to material between us and the radio source by verifying that the polarization angles have aλ 2 dependence and that the detected depolarization, if any, is consistent with that expected from instrumental effects (beam and bandwidth depolarization). This is the case for all of the radio galaxies analysed in this thesis and in most earlier work. This is consistent with the idea that radio sources have evacuated cavities in the IGM during their expansion (as observed in X-rays) and that there has been relatively little mixing of thermal and relativistic plasma. The apparent under-pressure in FR I lobes and tails is most easily explained if a small amount of thermal plasma has been entrained and heated, but the densities required are quite small, and consistent with the non-detection of internal Faraday rotation and depolarization at GHz frequencies. Prior to the work described here, it was generally thought that the observed RM was produced almost exclusively by the distributed hot component of the intra-group or intracluster medium and that interactions with the radio source were unimportant. In the present study, three distinct types of magnetic-field structure have been found: a component with large-scale fluctuations, indeed plausibly associated with the undisturbed intergalactic medium; a well-ordered field draped around the leading edges of radio lobes and a field with small-scale fluctuations in the shells of compressed gas surrounding the inner lobes, perhaps associated with a mixing layer. Foreground Faraday rotation in a spherically symmetrical, undisturbed IGM provides a natural explanation for the systematic asymmetry in RM variance and/or depolarization observed between the approaching and receding lobes (Laing 1988; Garrington et al. 1988; Morganti et al. 1997; Laing et al. 2008). In contrast, some authors (e.g. Bicknell, Cameron & Gingold 1990) have suggested that the RM could be localized in a thin layer around the source, but simple shell models of this type find difficulties in reproducing the observed asymmetries and are unlikely to be important on large scales. It is likely that the sphericallysymmetric group component dominates in large, tailed sources such as 3C 449. The results on banded RM structures presented in this thesis suggest that the magnetised medium producing them must have a scale comparable to that of the lobe width. If the 128
6.6. General conclusions 129 radio source is expanding supersonically, there will also be a bow shock ahead of the lobe in the hot gas which will naturally produce higher densities and stronger fields in the postshock material immediately surrounding the lobes. All of the sources which show banded RM structure show evidence of strong interaction with the surrounding medium, either from expansion driven by the jets (0206+35, 3C 270, 3C 353) or from X-ray observations (M 84). The best evidence for mixing between thermal and relativistic plasma comes from the observation of enhanced depolarization in the inner lobes of M 84, 3C 270 and 0755+37. It is in these regions that models like those of Bicknell, Cameron & Gingold (1990) are most likely to apply. 2. The connection between magnetic field and thermal gas The results of this work are consistent with the idea that the RM fluctuation amplitude in galaxy groups and clusters scales roughly linearly with density, ranging from a few rad m −2 in the sparsest environments (0755+37) through intermediate values of≈30-100 rad m −2 in groups (0206+35, 3C 270 and 3C 449) to∼ 10 4 rad m −2 in the centres of clusters with cool cores (M 87). The spatial variation of RM fluctuation amplitude in galaxy groups is consistent with a distribution for the magneto-ionic medium which is spherically symmetric, together with cavities coincident with the radio emission. The radial dependence of the field strength can be approximated by B(r)=〈B 0 〉 ( ) n e (r) η, n 0 with 1 > ∼ η> ∼ 0.5. The central magnetic-field strength B 0 is typically a fewµG and the magnetic field is never dynamically dominant over the modeled volumes. Thus the relation between magnetic field strength and density in galaxy groups appears to be a continuation of the trend observed in galaxy clusters (e.g. Brunetti et al. 2001; Dolag et al. 2006; Guidetti et al. 2008; Bonafede et al. 2010): the radial variations have similar functional forms and the normalization scales with density. 3. Magnetic-field structure In this thesis, two types of RM structures have been found: one with RM variations reasonably approximated as isotropic and random (3C 449, M 87, the western lobe of 3C 353) and one with two-dimensional banded RM patterns (0206+35, 3C 270, M 84, the eastern lobe of 3C 353 and perhaps 0755+37). The first type of structure is well reproduced by a Gaussian isotropic random magnetic field, as shown by earlier numerical modelling as well as that performed in this thesis. The second type shows evidence for a two-dimensional, ordered magnetic field. One of the major results of this thesis has been to demonstrate that RM enhancements are expected from compression of the surrounding magneto-ionic medium by expanding radio galaxies, but that such structures can have band-like shapes 129
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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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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 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: 6.5. Summary 127 responsible for th
Page 147 and 148: 6.7. Future prospects 131 and is th
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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