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98 5. Ordered magnetic fields around radio galaxies Figure 5.12: Geometry of the two-dimensional draped model for the magnetic field, described in Sec. 5.7.3. Panel (a): seen in the plane of the direction of the ambient magnetic field. Panel (b): seen in the plane perpendicular to the ambient field; only the innermost family of ellipses is plotted. For clarity, field lines behind the lobe are omitted. The colour of the curves indicates the strength of the field, decreasing from red to yellow. The crosses represent the radio core position. The coordinate system is the same as in Fig. 5.8 and the line of sight is in the X− Z plane. IMAGE: BLANK.FITS IMAGE: BLANK.FITS IMAGE: BLANK.FITS ;100 ;50 0 RAD/M/M rad m 0 10 20 30 40 RAD/M/M rad m −2 ;40 ;20 0 20 RAD/M/M rad m −2 x x x (a) (b) (c) Figure 5.13: Synthetic RM images produced for a two-dimensional draped model as described in Section 5.7.3. The field lines are a family of ellipses draped around the lobe as in Fig. 5.12. Panel (a): lobe inclined at 90 ◦ to the line-of-sight. (b) receding and (c) approaching lobes of a source inclined by 40 ◦ to the line-of-sight. The crosses and the arrows represent the radio core position and the lobe advance direction. prominent reversal is apparent in the receding lobe of 0206+35 (Fig. 5.2a) and multiple reversals across the eastern lobe of 3C 353 and in M 84 (Fig. 5.2b and d). The simplest way to reproduce these is to assume that the draped field also has reversals, presumably originating from a more complex initial field in the IGM. One realization of such a field configuration would be in the form of multiple toroidal eddies with radii smaller than the lobe size, as sketched in Fig. 5.14. Whatever the precise field geometry, I stress that the straightness of the observed multiple bands again requires a two-dimensional structure, with little dependence on Y. 98
5.8. Discussion 99 5.8 Discussion 5.8.1 Where do bands occur? The majority of published RM images of radio galaxies do not show bands or other kind of anisotropic structure, but are characterized by isotropic and random RM distributions (e.g. Laing et al. 2008, 3C 449, Chapter 4). On the other hand, I have presented observations of RM bands in four radio galaxies embedded in different environments and with a range of jet inclinations with respect to the line-of-sight. These sources are not drawn from a complete sample, so any quantitative estimate of the incidence of bands is premature, but it is possible to draw some preliminary conclusions. The simple two-dimensional draped-field model developed in Section 5.7.3 only generates RM bands when the line-of-sight intercepts the volume containing elliptical field lines, which happens for a restricted range of rotation around the source axis. At other orientations, the RM from this field configuration will be small and the observed RM may well be dominated by material at larger distances which has not been affected by the radio source. I therefore expect a minority of sources with this type of field structure to show RM bands and the remainder to have weaker, and probably isotropic, RM fluctuations. In contrast, the three-dimensional draped field model proposed by Dursi & Pfrommer (2008) predicts RM bands parallel to the source axis for a significant range of viewing directions: these have not (yet) been observed. The prominent RM bands here described occur only in lobed radio galaxies. In contrast, wellobserved radio sources with tails and plumes seem to be free of bands or anisotropic RM structure (e.g. 3C 31, 3C 449; Laing et al. 2008, Chapter 4). Furthermore, the lobes which show bands are all quite round and show evidence for interaction with the surrounding IGM. It is particularly striking that the bands in 3C 353 occur only in its eastern, rounded, lobe. The implication is that RM bands occur when a lobe is being actively driven by a radio jet into a region of high IGM density. Plumes and tails, on the other hand, are likely to be rising buoyantly in the group or cluster and I do not expect significant compression, at least at large distances from the nucleus. 5.8.2 RM bands in other sources The fact that RM bands have so far been observed only in a few radio sources may be a selection effect: much RM analysis has been carried out for galaxy clusters, in which most of the sources are tailed (e.g. Blanton et al. 2003). With a few exceptions like Cyg A (see below, Section 5.8.2), lobed FR I and FR II sources have not been studied in detail. 99
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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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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
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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
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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
Page 101 and 102: 5.4. Depolarization 85 Therefore, a
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Page 111 and 112: 5.7. Rotation-measure bands from a
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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 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
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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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