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6 1. Physics of radio galaxies 1.3 Radio galaxies Radio galaxies are radio-loud AGN hosted by massive early type galaxies (visual magnitude M v < -20), with radio powers at 408 MHz spanning the range 10 23−28 WHz −1 . Their radio spectra have approximately power-law forms with typical spectral indicesα=0.8±0.2, consistent with synchrotron emission from relativistic particles with power-law energy spectra (Sec. 1.2.1). The electron Lorentz factors areγ≥100 and magnetic field strengths are∼µG. The synchrotron origin of the radio emission is confirmed by the smooth broad-band nature of the emission (10 MHz- 10 GHz) and the high degree of linear polarization (commonly≥0.1 averaged across the source, but reaching values of 0.7 in sub-regions). The energy source for the relativistic particles and magnetic fields is thought to be the central black hole at the nucleus of the radio galaxy. The energy is transferred from this engine outwards by more or less collimated, initially relativistic, flows or beams, whose visible manifestations are the radio jets. 1.3.1 Radio-galaxy morphologies Radio galaxies show a wide range of structures and linear sizes, going from a few ten of pc up to Mpc (Bridle & Perley 1984; Laing 1993), and hence they can be more extended than the parent galaxy. The main morphological classification of extended radio galaxies is that made by (Fanaroff & Riley 1974). This was based on the sharp change in morphology of the sources in the 3C catalogue around the radio power 10 24.5 WHz −1 at 1.4 GHz, close to the break in the radio luminosity function. Fanaroff & Riley pointed out that low-power sources tend to be brightest close to their nuclei (“edge-darkened”) whereas high-power ones are brightest at their outer extremities (“edge-brightened”). These are respectively classified as Fanaroff-Riley I and Fanaroff-Riley II sources, (or FR I and FR II sources). The prototypes of FR I and FR II sources are respectively 3C 31 and Cygnus A (Fig. 1.1). The sources discussed in detail in this thesis are mostly members of the FR I class. The main morphological components of FR I and FR II radio galaxies observed with arcsecond resolution (e.g Bridle & Perley 1984) are as follows. • The “core” is an unresolved component coincident to within the observational errors with the optical nucleus. It represents the partially optically thick base of the jets (see below). Cores are relatively stronger in FR I sources. • The “jets” are long narrow features, which emerge from the core and propagate generally in opposite directions. FR I jets are typically bright, with wide opening angles, while those of 6
1.3. Radio galaxies 7 FR II sources appear weak and well collimated, suggesting a more efficient energy transport than in FR Is. FR I jets are thought to decelerate to sub-relativistic speeds on kpc scales, while FR II jets remain relativistic over their whole lengths. • The “hot-spots” are bright and compact regions observed only in FR II sources, close to the extremities of the radio structures. They are interpreted as the termination or major disruption of the jets at strong shocks (which requires that FR II jets are internally supersonic). • The “lobes” are wide structures with small axial ratios which lie on either side of the parent galaxy and are often aligned across the nucleus on scales up to Mpc. Most of the radio emission is therefore seen between the end of the jets and the core. Synchrotron spectra of lobes show steepening towards the nucleus and it is therefore likely that lobes are formed from relativistic particles left behind at or back-flowing from the region where the jet impacts the external medium. Lobes are found in both FR I and FR II sources. • The “tails” are elongated and sometimes irregular structures, mostly farther from the nucleus than the end of a jet. Their synchrotron spectra steepen away from the nucleus, suggesting that they are flowing outwards. They are found almost exclusively in FR I sources. 1.3.2 Morphology and expansion of lobes and tails The physics of the interaction between a radio source and the surrounding IGM appears to be significantly different for lobes and tails. Also, as will become apparent in later Chapters, the magnetization of the IGM around a radio source appears to depend on the morphology of the extended emission. For these reasons, I briefly discuss the differences between lobes and tails. The richness of the environment may play a role in the formation of tails rather than lobes. Most of the tailed sources are observed in galaxy clusters or rich groups and often show distorted morphologies which are likely to be caused by gas sloshing in the potential well of the cluster or by the high ram pressure exerted by the thermal gas on fast-moving galaxies (respectively Wideangle tails, e.g. 3C 465, Eilek et al. 1984 and Narrow-angle tails, e.g. NGC 1265, O’Dea & Owen 1986). There is therefore clear evidence for jet/environment interaction in tailed sources. Indeed, tails are likely to be formed if the jet entrains enough material to slow it to below the external sound speed and/or if buoyancy causes the synchrotron plasma to be pushed outwards. For both these reasons, FR I tails are thought to be expanding sub-sonically, basically buoyantly, and in pressure equilibrium with their surroundings. Lobes or bridges, on the other hand, do sometimes show evidence for shocks in the surrounding X-ray emitting gas, suggesting that they are expanding supersonically (Sec. 2.3.2). 7
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: 1.2. Non-thermal emission mechanism
Page 25 and 26: 1.3. Radio galaxies 9 (a) (b) Figur
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
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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
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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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4.7. 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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