Investigations of Faraday Rotation Maps of Extended Radio Sources ...

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16 CHAPTER 1. INTRODUCTION 1.3 The state of the Art - Cluster Magnetic Fields Clusters of galaxies are the largest gravitationally bound systems and the largest virialised structures in the Universe. At optical wavelengths, clusters appear as overdensities of galaxies with respect to the field average density. Apart from the galaxies, they contain a hot (T ≃ 10 8 K) and low density (n e ≃ 10 −3 cm −3 ) plasma – the intracluster medium (ICM). This plasma is observed through its luminous X-ray emission (L X ≃ 10 43 · · · 10 45 erg s −1 ) which is produced by thermal bremsstrahlung radiation. The ICM is magnetised which is directly demonstrated by the observation of largescale diffuse synchrotron sources as shown in Fig. 1.3. Relativistic electrons of the energy ≃ GeV spiralling around in µG magnetic fields emit synchrotron radiation in the radio wavelength range. For the observation of cluster magnetic fields, this diffuse large-scale synchrotron emission observed as radio haloes and relics (Sect. 1.3.1) and Faraday rotation measurements of points sources and individual extended radio sources (Sect. 1.3.2) in or behind cluster environments can be used. In the following, a short review is given about our knowledge of the magnetic field strength and structure in the ICM (for detailed reviews see Kronberg 1994; Carilli & Taylor 2002; Widrow 2002). 1.3.1 Radio Haloes and Relics Willson (1970) observed diffuse radio emission in the centre of the Coma cluster which could not be associated with any cluster source (see Fig. 1.3). A detailed study revealed that the emission is characterised by a steep spectral index. No structure on scales less than 30’ were detected. Willson (1970) proposed that the diffuse emission is synchrotron radiation. By applying equipartition arguments a field strength of 2 µG was deduced. Various sky surveys have increased the number of such large-scale diffuse sources to the present day (e.g. Giovannini et al. 1999; Hunstead & The Sumss Team 1999; Venturi et al. 2000; Kempner & Sarazin 2001). Still the number is quite small (∼ 50). However, a couple of properties have been derived for radio haloes. They are located in the cluster centre and are of roughly spherical shape extending about ∼ 1 Mpc. They are characterised by steep spectral indeces (α ≥ 1, if S ν ∝ ν −α ), by a low surface brightness (∼ 10 −6 Jy arcsec −2 at 1.4 GHz) and by low polarised flux (≤ 5%). This low degree of polarisation suggests strong depolarisation due to internal Faraday depolarisation since emission region and Faraday screen coincide. Since this sources are very extended, only low resolution observations are possible, therefore also beam depolarisation might play a role when the magnetic fields are disordered on smaller scales than the resolution. The low resolution observations prevent detailed investigations of the magnetic field structure on small scales. However, minimum/equipartition arguments for the synchrotron emission yield field strength estimates averaged over the radio halo volume (∼ 1 Mpc). For this analysis as described in Sect. 1.2.3, usually a low energy cut-off of 10 MHz, a high energy cut-off of 10 GHz are used and a scaling factor between relativistic proton and electron energy density k = 1 is assumed. These arguments lead to field strengths of 0.1 . . . 1µG (Feretti 1999). Another type of diffuse large-scale synchrotron sources are the radio relics. In
1.3. THE STATE OF THE ART - CLUSTER MAGNETIC FIELDS 17 Figure 1.3: A Westerbork Synthesis Radio Telescope map at 327 MHz of the Coma cluster from Giovannini et al. (1991). The central extended radio source C is a so called radio halo and a radio relic 1253+275 is observed at the periphery. The capital letters indicate extended Coma cluster galaxies, A = NGC 4839, B = NGC 4827, C = NGC 4789. 10’ correspond to 400 kpc h −1 50 contrast to haloes, radio relics are observed at the periphery of clusters and show elongated or irregular shape. They are strongly polarised which can be understood by low internal Faraday depolarisation due to the expected lower magnetic field strength and lower electron densities at the periphery of the clusters. The prototype for such a relic is the source 1253+275 at the periphery of the Coma cluster (see Fig. 1.3), which was first classified by Ballarati et al. (1981). At 20 cm, its polarisation is 25 . . . 30% and the magnetic field is oriented along the source major axis. Minimum energy arguments yield a magnetic field strength ∼ 0.55µG for the synchrotron emitting plasma (Giovannini et al. 1991). Another example of complex large-scale radio emission in a cluster can be found in Abell 2256 (Bridle & Fomalont 1976; Röttgering et al. 1994). Several head-tail radio galaxies, a central radio halo and two large relic sources are detected in this cluster. The relics are highly polarised, on average 20-30 % but values of up to 50 % are observed (Clarke & Enßlin 2001). The intrinsic magnetic field direction traces the bright filaments in the relics and it appears that there is a large scale order to the fields. Other large relic sources have been detected for instance in Abell 2255 (Burns et al. 1995; Feretti et al. 1997), in Abell 1300 (Reid et al. 1999), in Abell 2744 (Govoni et al. 2001a), in Abell 754 (Kassim et al. 2001) and in Abell 3667 (Röttgering et al. 1997; Johnston-Hollitt et al. 2003). In the latter cluster, there are two almost symmetric relics.
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118 CONCLUSIONS tually statisticall
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120 BIBLIOGRAPHY Conway, R. G. & St
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122 BIBLIOGRAPHY Ikebe, Y., Makishi
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124 BIBLIOGRAPHY Simard-Normandin,
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126 ACKNOWLEDGEMENTS in his long em
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Publications Journals 1. A Bayesian
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