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102 5. Ordered magnetic fields around radio galaxies IMAGE: ELLISSOIDE_COUNTER_40_1.8_ISO.FITS IMAGE: ELLISSOIDE_MAIN_40_1.8_ISO.FITS ;80 ;60 ;40 ;20 0 20 40 60 RAD/M/M rad m −2 (a) ;80 ;60 ;40 ;20 0 20 40 60 RAD/M/M rad m −2 (b) x x Receding IMAGE: COUNTER_40_2.2_KOLMO.FITS ;50 0 50 100 150 RAD/M/M rad m −2 (c) Approaching IMAGE: FMATH.FITS ;100 ;50 0 50 RAD/M/M−2 rad m (d) x x Receding Approaching Figure 5.15: Synthetic RM images of the lobes of 0206+35, produced by the sum of a draped field local to the source and an isotropic, random field spread through the group volume with the best-fitting power spectrum found in Sec. 5.5 (panels (a) and (b)) and with a Kolmogorov power spectrum (panels (c) and (d)). Both power spectra have the same high and low-frequency cut offs and the central magnetic field strengths are also identical. The crosses and the arrows represent the radio core position and the lobe advance direction. and assumed the same value for B 0 . Example RM images, shown in Figs. 5.15(a) and (b), should be compared with those for the draped field alone (Figs. 5.13b and c, scaled up by a factor of 1.8 to account for the difference in field strength) and with the observations (Fig.5.2a) after correction for Galactic foreground (Table 5.4). The model is self-consistent: the flat power spectrum found for 0206+35 does not give coherent RM structure which could interfere with the RM bands, which are still visible. I then replaced the isotropic component with one having a Kolmogorov power spectrum (q=11/3). I assumed identical minimum and maximum scales (2 and 40 kpc) as for the previous power spectrum and took the same central field strength (B 0 =1.8µG) and radial variation (Eq. 5.7). Example realizations are shown in Fig. 5.15(c) and (d). The bands are essentially invisible in the presence of foreground RM fluctuations with a steep power spectrum out to scales larger than their widths. It may therefore be that the RM bands in the sources here presented are especially prominent because the power spectra for the isotropic RM fluctuations have unusually low amplitudes and 102
5.8. Discussion 103 flat slopes. I have established these parameters directly for 0206+35, 3C 270 and 3C 353; M 84 is only 14 kpc in size and is located far from the core of the Virgo cluster, so it is plausible that the cluster contribution to its RM is small and constant. I conclude that the detection of RM bands could be influenced by the relative amplitude and scale of the fluctuations of the isotropic and random RM component compared with that from any draped field, and that significant numbers of banded RM structures could be masked by isotropic components with steep power spectra. 5.8.4 Asymmetries in RM bands The well-established correlation between RM variance and/or depolarization and jet sidedness observed in FR I and FR II radio sources is interpreted as an orientation effect: the lobe containing the brighter jet is on the near side, and is seen through less magneto-ionic material (e.g. Laing 1988; Morganti et al. 1997). For sources showing RM bands, it is interesting to ask whether the asymmetry is due to the bands or just to the isotropic component. In 0206+35, whose jets are inclined by≈40 ◦ to the line-of-sight, the large negative band on the receding side has the highest RM (Fig. 5.5). This might suggest that the RM asymmetry is due to the bands, and therefore to the local draped field. Unfortunately, 0206+35 is the only source displaying this kind of asymmetry. The other “inclined” source, M 84, (θ≈ 60 ◦ ) does not show such asymmetry: on the contrary the RM amplitude is quite symmetrical. In this case, however, the relation between the (well-constrained) inclination of the inner jets, and that of the lobes could be complicated: both jets bend by≈90 ◦ at distances of about 50 arcsec from the nucleus (Laing et al. in preparation), so that we cannot establish the real orientation of the lobes with respect to the plane of the sky. The low values of the jet/counter-jet ratios in 3C 270 and 3C 353 suggest that their axes are close to the plane of the sky, so that little orientation-dependent RM asymmetry would be expected. Indeed, the lobes of 3C 270 show similar RM amplitudes, while the large asymmetry of RM profile of 3C 353 is almost certainly due to a higher column density of thermal gas in front of the eastern lobe. Within this small sample, there is therefore no convincing evidence for higher RM amplitudes in the bands on the receding side, but neither can such an effect be ruled out. In models in which an ordered field is draped around the radio lobes, the magnitude of any RM asymmetry depends on the field geometry as well as the path length (cf. Laing et al. 2008 for the isotropic case). For instance, the case illustrated in Figs 5.13(b) and (c) shows very little asymmetry even forθ= 40 ◦ . The presence of a systematic asymmetry in the banded RM component could therefore be used to constrain the geometry. 103
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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
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 and 106: 5.6. Rotation-measure bands from co
Page 111 and 112: 5.7. Rotation-measure bands from a
Page 113 and 114: 5.7. Rotation-measure bands from a
Page 115 and 116: 5.8. Discussion 99 5.8 Discussion 5
Page 117: 5.8. Discussion 101 line−of−sig
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 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 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,
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152 BIBLIOGRAPHY 152
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