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10 1. Physics of radio galaxies medium (Chapter 2) is useful to understand the equilibrium conditions of radio galaxies. In both FR I and FR II radio sources, B eq and P syn are found to be a fewµG and 10 −12 −10 −13 dyne/cm 2 , respectively. However, all the equipartition estimates must be taken with caution, because each of the variables in Eqs. 1.11 and 1.12 is potentially a source of uncertainty. Indeed, the filling factor Φ is unknown and often assumed to be 1 although in some radio galaxies there is evidence for filamentary structure. Some assumptions on the geometry of the source must also be made in order to compute the source volume V. The ratioζ is also subject to major uncertainties, since the energy density in relativistic protons is unconstrained. Moreover, the equipartition parameters are strongly related to the functional form of the particle energy spectrum and therefore to the synchrotron emission spectrum. Good constraints on the energy spectrum require wide frequency coverage. Indeed, even small changes in the lower limit of the relativistic particle energy can have a huge impact on the determination of B eq and P min . This is particularly true for steep radio spectra, where low energy particles contribute most of the total energy. My personal conclusion is that minimum energy arguments are of limited usefulness. The best use of these estimates lies in making the most conservative assumptions and treating the resulting pressures as lower limits. 1.3.5 Field and particle content of radio lobes and tails FR I radio galaxies with tails are believed to expand sub-sonically in pressure equilibrium with the surroundings, and to rise buoyantly in the intergalactic medium. On the other hand, some FR I sources with bridges appear to be surrounded by shocked X-ray emitting gas, which suggests supersonic expansion. As already pointed out, when IC emission is detected from the radio sources, then the total energy density in relativistic electrons and fields can be estimated and compared with the external pressure, also derived from X-ray observations. For some FR II sources, this has been done (e.g. Kataoka & Stawarz 2005; Croston et al. 2004; Laskar et al. 2009; Isobe et al. 2011) The conclusion is that most FR II lobes are close to the minimum energy condition, in the sense that relativistic electron and field energy densities are comparable, and that their sum is in turn close to the external pressure, as expected for static thermal confinement (e.g. Hardcastle et al. 2002; Croston et al. 2004; Konar 2009). So far, there are no unambiguously detections of IC emission from FR I lobes. Instead, several studies have shown that the synchrotron equipartition pressure P syn within the FR I radio lobes is often an order of magnitude smaller than the external pressure exerted by the hot surrounding gas (e.g. Morganti et al. 1988; Worrall & Birkinshaw 2000; Blanton et al. 2001; de Young 2006; Croston et al. 2003, 2008). Without a further source of internal 10
1.3. Radio galaxies 11 pressure, the radio sources would then collapse. To solve the pressure balance problem for FR I sources we need to consider: 1. deviations from equipartition or 2. an additional source of pressure which is not detectable by current radio or X-ray observations. Deviations from equipartition in the sense of electron dominance could be given by a large excess of low-energy electrons. These would be detectable via their IC radiation in at least some cases (e.g. Croston, Hardcastle & Birkinshaw 2005). Therefore deviations from equipartition would have to be in the direction of magnetic dominance. An additional source of pressure could be provided by relativistic protons. These would have to be accelerated in the lobes, since a relativistic proton population in the jets would exert too high a pressure to allow collimation. Even cold protons cannot be transported by the jets in large enough numbers without violating constraints on the mass flux (Laing & Bridle 2002a). The best candidate to solve the pressure balance is therefore heated and entrained thermal material. This would have to be hot (kT≥10 keV, e.g. Nulsen et al. 2002) but tenuous, to provide enough pressure without radiating sufficiently to erase the soft X-ray depressions (“cavities”), often observed at the position of radio lobes (Sec. 2.3.1). 11
Page 1: Alma Mater Studiorum Università de
Page 5: To Hypatia from Alexandria in Egypt
Page 9 and 10: Contents Abstract i 1 Physics of ra
Page 11 and 12: 4.6.3 The outer scale of the magnet
Page 13 and 14: Abstract The existence of diffuse m
Page 15 and 16: 2. Two-dimensional Monte Carlo simu
Page 17 and 18: Chapter 1 Physics of radio galaxies
Page 19 and 20: 1.2. Non-thermal emission mechanism
Page 21 and 22: 1.2. Non-thermal emission mechanism
Page 23 and 24: 1.3. Radio galaxies 7 FR II sources
Page 25: 1.3. Radio galaxies 9 (a) (b) Figur
Page 29 and 30: Chapter 2 The X-ray environment of
Page 31 and 32: 2.2. The thermal component 15 Figur
Page 33 and 34: 2.3. Radio source - environment int
Page 35 and 36: 2.3. Radio source - environment int
Page 37 and 38: Chapter 3 Magnetic fields in the ho
Page 39 and 40: 3.1. Introduction 23 Figure 3.1: Ex
Page 41 and 42: 3.1. Introduction 25 to the relativ
Page 43 and 44: 3.2. Faraday Rotation 27 The RM can
Page 45 and 46: 3.2. Faraday Rotation 29 Figure 3.4
Page 47 and 48: 3.3. Depolarization 31 3.3 Depolari
Page 49 and 50: 3.5. The magnetic field profile 33
Page 51 and 52: 3.6. Tangled magnetic field 35 rad
Page 53 and 54: 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
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4.6. Three-dimensional analysis 61
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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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5.1. The Sample 77 5.1.1 0206+35 02
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5.2. Analysis of RM and depolarizat
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5.3. Rotation measure images 81 IMA
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5.3. Rotation measure images 83 cor
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5.4. Depolarization 85 Therefore, a
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5.5. Rotation measure structure fun
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5.6. Rotation-measure bands from co
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5.7. Rotation-measure bands from a
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5.7. Rotation-measure bands from a
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5.8. Discussion 99 5.8 Discussion 5
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5.8. Discussion 101 line−of−sig
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5.8. Discussion 103 flat slopes. I
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5.9. Conclusions and outstanding qu
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5.9. Conclusions and outstanding qu
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Chapter 6 Faraday rotation in two e
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6.2. Two-dimensional analysis: rota
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6.3. Two-dimensional analysis: stru
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6.4. Preliminary conclusions 121 Ta
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6.4. Preliminary conclusions 123
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Summary and conclusions The goal of
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6.5. Summary 127 responsible for th
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6.6. General conclusions 129 radio
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6.7. Future prospects 131 and is th
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Appendix 133
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136 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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