Origin of Galactic and Extragalactic Magnetic Fields(6)

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

that the magnetic eld is symmetric about the equatorial plane, i.e., an even-parity axisymmetric (S0) con guration (Han, Beck, & Berkhuijsen 1998). Among S0-type galaxies, there is an additional question as to the direction of the magnetic eld, namely whether the eld is oriented inward toward the center of the galaxy or outward (Krause & Beck 1998). The two possibilities can be distinguished by comparing the sign of the RM (as a function of position on the disk) with velocity eld data. Krause & Beck (1998) point out that in four of ve galaxies where the eld is believed to be axisymmetric, those elds appear to be directed inward. This result is somewhat surprising given that a magnetic dynamo shows no preference for one type of orientation over the other. It would be premature to draw conclusions based on such a small sample. Nevertheless, if, as new data becomes available, a preference is found for inward over outward directed elds (or more realistically, a preference for galaxies that are in the same region of space to have the same orientation), it would reveal a preference in initial conditions and therefore speak directly to the question of seed elds.3. Connection with Spiral StructureOften, the spiral magnetic structures detected in disk galaxies appear to be closely associated with the material spiral arms. A possible connection between magnetic and optical spiral structure was rst noticed in observations of M83 (Sukumar & Allen 1989), IC 342 and M81 (Krause, Hummel, & Beck 1989a, 1989b). A particularly striking example of magnetic spiral structure is found in the galaxy NGC 6946, as shown in Figure 3 (Beck & Hoernes 1995; Frick et al. 2000). In each case, the map of linearly polarized synchrotron emission shows clear evidence for spiral magnetic structures across the galactic disk. The magnetic eld in IC 342 appears to be an inwardly-directed axisymmetric spiral while the eld in M81 is more suggestive of a bisymmetric con guration (Sofue, Takano, &13FIG. 2 Field lines for even (top panel) and odd (bottom panel) con gurations. Shown are cross-sections perpendicular to the equatorial plane and containing the symmetry axis of the galaxy (i.e., poloidal planes). The toroidal eld is indicated by an ‘x’ ( eld out of the page) or ‘dot’ ( eld into the page).Fujimoto 1980; Krause, Hummel, & Beck 1989a, 1989b; Krause 1990). In many cases, magnetic spiral arms are strongest in the regions between the optical spiral arms but otherwise share the properties (e.g., pitch angle) of their optical counterparts. These observations suggest that either the dynamo is more e cient in the interarm regions or that magnetic elds are disrupted in the material arms. For example, Mestel & Subramanian (1991, 1993) proposed that the α-e ect of the standard dynamo contains a non-axisymmetric contribution whose con guration is similar to that of the material spiral arms. The justi cation comes

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

from one version of spiral arm theory in which the material arm generates a spiral shock in the interstellar gas. The jump in vorticity in the shock may yield an enhanced αe ect with a spiral structure. Further theoretical ideas along these lines were developed by Shukurov (1998) and a variety of numerical simulations which purport to include nonaxisymmetric turbulence have been able to reproduce the magnetic spiral structures found in disk galaxies (Rohde & Elstner 1998; Rohde, Beck, & Elstner 1999; Elstner, Otmianowska-Mazur, von Linden, & Urbanik 2000). Along somewhat di erent lines, Fan & Lou (1996) attempted to explain spiral magnetic arms in terms of both slow and fast magnetohydrodynamic waves. Recently, Beck et al. (1999) discovered magnetic elds in the barred galaxy NGC 1097. Models of barred galaxies predict that gas in the region of the bar is channeled by shocks along highly non-circular orbits. The magnetic eld in the bar region appears to be aligned with theoretical streamlines suggesting that the eld is mostly frozen into the gas ow in contrast with what is expected for a dynamo-generated eld. The implication is that a dynamo is required to generate new eld but that inside the bar simple stretching by the gas ow is the dominant process (see Moss et al. 2001).14FIG. 3 Polarized synchrotron intensity (contours) and magnetic eld orientation of NGC 6946 (obtained by rotating E-vectors by 90 ) observed at λ6.2 cm with the VLA (12.5 arcsec synthesized beam) and combined with extended emission observed with the E elsberg 100 m telescope (2.5 arcmin resolution). The lengths of the vectors are proportional to the degree of polarization. (From Beck & Hoernes 1996.)4. Halo FieldsRadio observations of magnetic elds in edge-on spiral galaxies suggest that in most cases the dominant component of the magnetic eld is parallel to the disk plane (Dumke, Krause, Wielebinski, & Klein 1995). However, for at least some galaxies, magnetic elds are found to extend well away from the disk plane and have strong vertical components. Hummel, Beck, and Dahlem (1991) mapped two such galaxies, NGC 4631 and NGC 891, in linearly polarized radio emission and found elds with strength ～ 5 and ～ 8 µG respectively with scale heights ～ 5 10 kpc. The elds in these two galaxies have rather di erent characteristics: In NGC 4631 (Figure 4), numerous prominent radio spurs are found throughout the h …… 此处隐藏：7502字，全部文档内容请下载后查看。喜欢就下载吧 ……