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114 6. Faraday rotation in two extreme environments At 4.0 arcsec the RM distribution has a mean of−5.4 rad m −2 withσ RM =5.1 rad m −2 and an average fitting errorσ RMfit σ RM =6.0 rad m −2 andσ RMfit = 2.0 rad m −2 . = 0.8 rad m −2 . At 1.3 arcsec the mean RM is−0.1 rad m −2 with The small amplitude of the RM fluctuations is consistent with an origin in the IGM of the isolated host galaxy, rather than the atmosphere of a more extended group. There is a clear asymmetry in RM structure between the E and W lobes at both resolutions. Firstly, most of the E (approaching) lobe is characterized by a fairly uniform, positive RM, and the largest RM fluctuations, of both signs, are seen in the W (receding) lobe. Secondly, both lobes show anisotropic features, but with different characteristics. In the E lobe there is a thin, straight structure elongated parallel to the source axis with almost zero〈RM〉 . This “stripe” is very striking on the 1.3 arcsec map and appears to be almost coincident with the main jet axis, extending beyond the apparent termination of the jet as far as the boundary of the lobe. It is probably the clearest known example of kpc-scale, jet-related RM structure. It can be interpreted in at least two possible ways: 1. the jet is mostly in front of the Faraday screen. This might happen if the jet has displaced a relatively large amount of hot gas or if it is bending along the line-of-sight. 2. The jet lies behind a Faraday screen with an intrinsically low RM along the stripe, i.e., the magnetic field is feeble and/or preferentially aligned in the plane of the sky on the projected jet axis. The latter point requires that the Faraday screen somehow knows about the jet. The 4.0-arcsec RM map reveals arc-like features at the leading edge of the W lobe. These have alternating signs and must therefore be associated with field reversals. They are confirmed by the higher resolution map, where the iso-RM contours appear to be fairly straight and perpendicular to the jet axis. The resemblance to the RM bands discussed in Chapter 5 is obvious. It is possible that these features are also caused by draping of the magnetic field around the leading edge of the lobe. The Burn law k maps are shown in Fig. 6.5. At 4.0 arcsec resolution, the mean value of k is 106 rad 2 m −4 , corresponding to a depolarization DP 21cm 6cm = 0.79. The mean observed k values for sub-regions of 0755+37 at both the angular resolutions are listed in Table 6.4. From this table, but also by visual inspection of Fig. 6.5 (left), it is evident that at 4.0 arcsec the highest depolarization is observed in the W (receding) lobe. This is consistent with a higher path length through the X-ray emitting gas (Laing-Garrington effect; Laing 1988; Garrington et al. 1988). In contrast, the Burn law k distribution at 1.3 arcsec is very symmetrical and has a much lower overall mean, implying that the RM fluctuations are mostly resolved, consistent with a foreground screen. 114
6.2. Two-dimensional analysis: rotation measure and depolarization 115 IMAGE: 0755.RM.4.0_BLANK.FITS DECLINATION (J2000) 37 48 00 37 47 30 37 47 00 37 46 30 ;20 ;10 0 10 20 RAD/M/M 07 58 36 07 58 32 07 58 28 07 58 24 RIGHT ASCENSION (J2000) 10 kpc DECLINATION (J2000) 37 48 00 37 47 30 37 47 00 37 46 30 IMAGE: 0755.RM.1.3_BLANK.FITS ;20 ;10 0 10 20 RAD/M/M 07 58 36 07 58 32 07 58 28 07 58 24 RIGHT ASCENSION (J2000) 10 kpc 07 58 20 Figure 6.4: Images of the rotation measure for 0755+37, computed from weighted least-squares fits to the position angle –λ 2 relation at frequencies of 1385.1, 1469.1 and 4860.1 MHz. The angular resolutions are 4.0 arcsec FWHM (left) and 1.3 arcsec FWHM (right). IMAGE: 0755_K;BURN_4ARCSEC_4SIGMAP.FITS 0 100 200 300 400 rad^2/m^4 IMAGE: 0755_K;BURN_1.3ARCSEC.FITS 0 100 200 300 400 rad^2/m^4 37 48 00 37 48 00 DECLINATION (J2000) 37 47 30 37 47 00 37 46 30 DECLINATION (J2000) 37 47 30 37 47 00 37 46 30 07 58 36 07 58 32 07 58 28 07 58 24 RIGHT ASCENSION (J2000) 07 58 36 07 58 32 07 58 28 07 58 24 RIGHT ASCENSION (J2000) 07 58 20 Figure 6.5: Images of Burn law k for 0755+37 computed from weighted least-squares fits to the relation p(λ)= p(0)exp(−kλ 4 ) at frequencies of 1385.1, 1469.1 and 4860.1 MHz. The angular resolutions are 4.0 arcsec FWHM (left) and 1.3 arcsec FWHM (right). This scenario is confirmed by the profiles of〈k〉 across the source (Fig. 6.6): at lower resolution the profile is asymmetrical, with less depolarization at a given distance from the core in the E lobe and a peak which is displaced slightly W of the nucleus. In contrast, the〈k〉 profile at 1.3 arcsec resolution is symmetrical and lies significantly below that at 4.0 arcsec. The high k values in the W lobe at the lower resolution are mostly seen in a cone-like structure (Fig. 6.5 left), as was already pointed out by Bondi et al. (2000). It is possible that the origin of such structure may be gas and field compression or the development of a mixing layer as discussed for M 84 and 3C 270 (Sec. 5.8.5). To confirm this picture, higher resolution X-ray observations are needed. In contrast, a region with very low and almost constant depolarization is observed at the leading edge of the W lobe, beyond the end of the approaching jet. 115
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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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4.6. Three-dimensional analysis 61
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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 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,
Page 168 and 169: 152 BIBLIOGRAPHY 152
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