Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf

More documents

Recommendations

Info

8 1. Physics of radio galaxies So far, the best examples of strong shocks surrounding bridges are found only in two FR I sources (Centaurus A, Kraft et al. 2003; NGC 3801, Croston et al. 2007). Instead, weak shocks appear to occur more frequently and they are seen in both FR I (e.g. Perseus, Fabian et al. 2003; Hydra A, Nulsen et al. 2005a; M87, Forman et al. 2005) and FR II sources (e.g. Cygnus A, Wilson, Smith & Young 2006; Hercules A, Nulsen et al. 2005b; 3C 444, Croston et al. 2010). Therefore, standard models of supersonic radio-lobe expansion, which were originally thought to apply only to FR II sources (e.g. Scheuer 1974; Kaiser & Alexander 1997), are also likely to describe lobes in FR I’s. It may be that the expansion of FR I lobes is supersonic only in the forward direction (driven by the ram pressure of the jets) and that the lobes are in static pressure equilibrium with their surroundings closer to the galaxy. To conclude, the factors which determine the large-scale morphology of radio galaxies have not been disentangled yet, although the initial speeds of the jets and their density contrast with the external medium both seem to be important. Whether a lobe or a tail is formed clearly makes a substantial difference to the interaction between a source and its surroundings. 1.3.3 Internal physical conditions of radio galaxies In order to understand the interaction of radio galaxies with their environments, we need to quantify the internal conditions in the extended emitting regions (lobes and tails). From the synchrotron emissivity alone, it is not possible to derive unambiguously the energy of the relativistic particles and magnetic field, because of the degeneracy between them (Eq. 1.1). Constraints on the energy density in relativistic electrons and magnetic field can be derived from synchrotron emission if IC emission from the same electron population is observed, in which case the degeneracy between particles and fields can be resolved (Eq. 1.10). The sum of particle and field energy densities can then be compared with the thermal pressure in the surroundings. If IC emission is lacking, in order to separate the magnetic and relativistic particles densities, one must make some assumptions about the relation between electrons and magnetic field. One commonly used approach is to assume energy equipartition between them: this is described in the next section. 1.3.4 Equipartition parameters In the context of radio sources, the term “energy equipartition” is commonly used to mean that the energy densities in relativistic particles and magnetic field are equal. This is an assumption, with no rigorous justification, but has some empirical support from observations of FR II lobes. It is also close to the condition that the total energy density in particles and field is a minimum. 8
1.3. Radio galaxies 9 (a) (b) Figure 1.1: Examples of extended radio galaxies with lobes or plumes. Top panel: bridged radio galaxy Cygnus A (FR II) in a cluster of galaxy at z=0.056. This is the brightest extragalactic radio source in the sky. Bottom panels: montage by A.H. Bridle showing the tailed radio source 3C 31 (FR I, in red) at the center of a galaxy group at z=0.0169, superimposed on Hubble Space Telescope WFPC2 image (a) and on DSS image (b). The field strength and pressure corresponding to the equipartition condition (e.g. Longair 1994) are: B eq [µ G] ≃ 0.9·L 2/7 [ergs −1 ] V−2/7 [kpc 3 ] ζ2/7 Φ −2/7 (1.11) P eq [dyne/cm 2 ]∝L 4/7 [ergs −1 ] V−4/7 [kpc 3 ] ζ4/7 Φ −4/7 , (1.12) where L and V are the radio luminosity and the volume of the source,ζ is the ratio of the total energy in particles to the energy in electrons alone, andΦis the so called “filling factor”, which is the fraction of the total source volume occupied by emitting material. For a relativistic plasma Γ=4/3, so the total minimum energy density of the source is u min ≈ 3× P eq . Since P eq represents the pressure exerted by the synchrotron-emitting plasma, throughout the thesis it will be reported as P syn . The comparison of the pressure P eq with the external one provided by the surrounding 9
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: 1.3. Radio galaxies 7 FR II sources
Page 27 and 28: 1.3. Radio galaxies 11 pressure, th
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
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
4.7. Summary and comparison with ot
Page 85 and 86:
Page 87 and 88:
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 107 and 108:
Page 109 and 110:
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 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 129 and 130:
Page 131 and 132:
Page 133 and 134:
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:
Page 156 and 157:
Page 158 and 159:
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
show all

Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf

Create successful ePaper yourself

Delete template?

Save as template?