Origin of Galactic and Extragalactic Magnetic Fields(5)

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength (10^{-7}-10^{-5} G) and huge spatial scale (~Mpc). It is generally assumed that magnetic fields in s

erved including NGC 6946 (Fendt, Beck, & Neininger 1998), M51 (Scarrott, Ward-Thompson, & Warren-Smith 1987), and NGC 1068 (Scarrott et al. 1991). The optical polarization map of M51, for example, suggests that its magnetic eld takes the form of an open spiral which extends from within 200 pc of the galactic center out to at least 5 kpc. Radio polarization data also indicates a spiral structure for the magnetic eld for this galaxy providing information on the magnetic con guration from 3 kpc to 15 kpc (Berkhuijsen et al. 1997). Nevertheless, it is sometimes di cult to reconcile the optical and radio data. Over much of the M51 disk, the data indicates that the same magnetic eld gives rise to radio synchrotron emission and to the alignment of dust grains (Davis-Greenstein mechanism). However in one quadrant of the galaxy, the direction of the derived eld lines di er by ～ 60 suggesting that either the magnetic elds responsible for the radio and optical polarization reside in di erent layers of the ISM or that the optical polarization is produced by a mechanism other than the alignment of dust grains by the magnetic eld.B. Spiral GalaxiesSpiral galaxies are a favorite laboratory for the study of cosmic magnetic elds. There now exist estimates for the magnetic eld strength in well over 100 spirals and, for a sizable subset of those galaxies, detailed studies of their magnetic structure and morphology.1. Field StrengthThe magnetic eld of the Galaxy has been studied through synchrotron emission, Faraday rotation, optical polarization, and Zeeman splitting. The latter provides a direct determination of the in situ magnetic eld at speci c sites in the Galaxy. Measurements of the 21-cm Zeeman e ect in Galactic HI regions reveal regular magnetic elds11 with B 2 10 µG, the higher values being found in dark clouds and HI shells (Heiles 1990 and references therein). Similar values for the Galactic eld have been obtained from Faraday rotation surveys of galactic and extragalactic sources (i.e., estimates of RMg ). Manchester (1974) has compiled RM data for 38 nearby pulsars and was able to extract the Galactic contribution. He concluded that the coherent component of the local magnetic eld is primarily toroidal with a strength B 2.2 ± 0.4 µG. Subsequent RM studies con rmed this result and provided information on the global structure of the Galactic magnetic eld (see for example Rand & Lyne 1994 and also Frick, Stepanov, Shukurov, & Sokolo (2001) who describe a new method for analysing RM data based on wavelets). Early estimates of the strength of the magnetic eld from synchrotron data were derived by Phillipps et al. (1981). Their analysis was based on a model for Galactic synchrotron emission in which the magnetic eld in the Galaxy is decomposed into regular and tangled components. An excellent t to the data was obtained when each component was assumed to have a va

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength (10^{-7}-10^{-5} G) and huge spatial scale (~Mpc). It is generally assumed that magnetic fields in s

lue of 3 µG. More recent estimates give ～ 4µG for the regular and ～ 6µG for the total local eld strength (Beck 2002). Magnetic elds in other galaxies are studied primarily through synchrotron and Faraday rotation observations. An interesting case is provided by M31. Polarized radio emission in this galaxy is con ned to a prominent ring ～ 10 kpc from the galaxy’s center. The equipartition eld strength in the ring is found to be ～ 4 µG for both regular and random components. Fitt & Alexander (1993) applied the minimum energy method to a sample of 146 late-type galaxies. The distribution of eld strengths across the sample was found to be relatively narrow with an average value of Beq 11 ± 4 µG (using k = 100), in agreement with earlier work by Hummel et al. (1988). The magnetic eld strength does not appear to depend strongly on galaxy type although early-type galaxies have a slightly higher mean. A few galaxies have anomalously strong magnetic elds. A favorite example is M82 where the eld strength, derived from radio continuum observations, is 50 µG (Klein, Wielebinski, & Morsi 1988). This galaxy is characterized by an extraordinarily high star formation rate.2. Global Structure of the Magnetic Field in SpiralsAnalysis of RM data as well as polarization maps of synchrotron emission can be used to determine the structure of magnetic elds in galaxies. It is common practice to classify the magnetic eld con gurations in disk galaxies according to their symmetry properties under rotations about the spin axis of the galaxy. The simplest examples are the axisymmetric and bisymmetric spiral patterns shown in Figure 1. In principle, an RM map can distinguish between the di erent possibilities (Tosa & Fujimoto 1978; Sofue, Fujimoto, & Wielebinski 1986). For example, one can plot RM as a function of the azimuthal angle φ at xed physical distance from the galactic center. The result will be a single (double) periodic distribution for a pure axisymmetric (bisymmetric) eld con guration. The RM-φ method has a number of weaknesses as outlined in Ruzmaikin, Sokolo , Shukurov, & Beck (1990) and Sokolo , Shukurov, & Krause (1992). In particular, the method has di culty disentangling a magnetic eld con guration that consists of a superposition of di erent modes. In addition, determination of the RM is plagued by the “nπ degeneracy” and therefore observations at a number of w …… 此处隐藏：7444字，全部文档内容请下载后查看。喜欢就下载吧 ……