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90 5. Ordered magnetic fields around radio galaxies IMAGE: K;BURN_3p_4.5_BLANKED.FITS CNTRS: M84_CHANDRA_NOPTS_AS_1.3_4.5.FITS.3 0 100 200 300 400 500 600 IMAGE: 3C270_KBURN_5.0.FITS CNTRS: XMM_12_ARCSEC_AS_1.3_5.0.FITS rad^2/m^4 m 0 100 200 300 400 500 600 rad^2/m^4 rad m 12 54 00 05 52 00 DECLINATION (J2000) 12 53 00 DECLINATION (J2000) 05 50 00 05 48 00 12 52 00 (a) 12 25 09 12 25 06 12 25 03 RIGHT ASCENSION (J2000) 12 25 00 (b) 12 19 40 12 19 30 12 19 20 RIGHT ASCENSION (J2000) 12 19 10 Figure 5.6: Burn law k images of M 84 (a) and 3C 270 (b) overlaid on X-ray contours derived from Chandra and XMM-Newton data, respectively. with the thermal gas, displacing rather than mixing with it (see McNamara & Nulsen 2007 for a review). For the sources here presented, the X-ray observations of M 84 (Finoguenov et al. 2008, Fig.5.1b) and 3C 270 (Croston et al. 2008, Fig.5.1c) show cavities and arcs of enhanced brightness, corresponding to shells of compressed gas bounded by weak shocks. The strength of any pre-existing field in the IGM, which will be frozen into the gas, will also be enhanced in the shells. Therefore a significant enhancement in RM is expected. A more extreme example of this effect will occur if the expansion of the radio source is highly supersonic, in which case there will be a strong bow-shock ahead of the lobe, behind which both the density and the field become much higher. Regardless of the strength of the shock, the field is modified so that only the component in the plane of the shock is amplified and the post-shock field tends become ordered parallel to the shock surface. The evidence so far suggests that shocks around radio sources of both FR classes are generally weak (e.g. Forman et al. 2005, Wilson, Smith & Young 2006, Nulsen et al. 2005a). There are only two examples in which highly supersonic expansion has been inferred: the southern lobe of Centaurus A (M≈8; Kraft et al. 2003) and NGC 3801 (M≈4; Croston et al. 2007). There is no evidence that the sources here described are significantly over-pressured compared with the surrounding IGM (indeed, the synchrotron minimum pressure is systematically lower than the thermal pressure of the IGM; Table 5.2). The sideways expansion of the lobes is therefore unlikely to be highly supersonic. The shock Mach number estimated for all the sources from ram pressure balance in the forward direction is also≈1.3. This estimate is consistent with that for M 84 made by Finoguenov et al. (2006) and also with the lack of detection of strong shocks in the X-ray data 90
5.6. Rotation-measure bands from compression 91 (a) (b) (c) (d) Figure 5.7: Plots of the RM structure functions for the isotropic sub-regions of 0206+35,3C 353 and 3C 270 described in the text. The horizontal bars represent the bin width and the crosses the centroids for data included in the bins. The red lines are the predictions for power law power spectra, including the effects of the convolving beam. The vertical error bars are the rms variations for the structure functions derived for multiple realizations of the data. The fitted structure functions for 3C 270 are derived for the same power spectrum parameters. for the other sources. In this section, I investigate how the RM could be affected by compression. I consider a deliberately oversimplified picture in which the radio source expands into an IGM with an initially uniform magnetic field, B. This is the most favourable situation for the generation of large-scale, anisotropic RM structures: in reality, the pre-existing field is likely to be highly disordered, or even isotropic, because of turbulence in the thermal gas. I stress that I have not tried to generate a self-consistent model for the magnetic field and thermal density, but rather to illustrate the generic effects of compression on the RM structure. In this model the radio lobe is an ellipsoid with its major axis along the jet and is surrounded by a spherical shell of compressed material. This shell is centred at the mid-point of the lobe (Fig. 5.8) and has a stand-off distance equal to 1/3 of the lobe semi-major axis at the leading 91
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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
Page 55 and 56: 3.7. The magnetic field power spect
Page 57 and 58: 3.7. The magnetic field power spect
Page 59 and 60: Chapter 4 Structure of the magneto-
Page 61 and 62: 4.1. The radio source 3C 449: gener
Page 63 and 64: 4.3. The Faraday rotation in 3C 449
Page 65 and 66: 4.3. The Faraday rotation in 3C 449
Page 67 and 68: 4.4. Depolarization 51 Figure 4.4:
Page 69 and 70: 4.4. Depolarization 53 1.25 arcsec
Page 71 and 72: 4.5. Two dimensional analysis 55 sm
Page 73 and 74: 4.5. Two dimensional analysis 57 Fi
Page 75 and 76: 4.6. Three-dimensional analysis 59
Page 83 and 84: 4.7. Summary and comparison with ot
Page 89 and 90: Chapter 5 Ordered magnetic fields a
Page 91 and 92: 5.1. The Sample 75 35 49 00 ROSAT P
Page 93 and 94: 5.1. The Sample 77 5.1.1 0206+35 02
Page 95 and 96: 5.2. Analysis of RM and depolarizat
Page 97 and 98: 5.3. Rotation measure images 81 IMA
Page 99 and 100: 5.3. Rotation measure images 83 cor
Page 101 and 102: 5.4. Depolarization 85 Therefore, a
Page 103 and 104: 5.5. Rotation measure structure fun
Page 105: 5.6. Rotation-measure bands from co
Page 109 and 110: 5.6. Rotation-measure bands from co
Page 111 and 112: 5.7. Rotation-measure bands from a
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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
Page 123 and 124: 5.9. Conclusions and outstanding qu
Page 125 and 126: Chapter 6 Faraday rotation in two e
Page 127 and 128: 6.2. Two-dimensional analysis: rota
Page 135 and 136: 6.3. Two-dimensional analysis: stru
Page 137 and 138: 6.4. Preliminary conclusions 121 Ta
Page 139 and 140: 6.4. Preliminary conclusions 123
Page 141 and 142: Summary and conclusions The goal of
Page 143 and 144: 6.5. Summary 127 responsible for th
Page 145 and 146: 6.6. General conclusions 129 radio
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 154 and 155: 138 Data reduction and imaging of l
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140 Data reduction and imaging of l
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142 Data reduction and imaging of l
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144 BIBLIOGRAPHY Bîrzan, L., Raffe
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146 BIBLIOGRAPHY Ensslin, T.A. & Go
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148 BIBLIOGRAPHY Jeltema, T.E. & Pr
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150 BIBLIOGRAPHY Owen, F.N., 1989,
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152 BIBLIOGRAPHY 152
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