Origin of Galactic and Extragalactic Magnetic Fields(7)

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

measures that di er by 100 rad m 2 (Green eld, Roberts, & Burke 1985). The conjecture is that light for one of the images passes through a giant cD elliptical galaxy whose magnetic eld is responsible for the observed Faraday rotation. A more detailed review of the observational literature can be found in Moss & Shukurov (1996). These authors stress that while the evidence for microgauss elds in ellipticals is strong, there are no positive detections of polarized synchrotron emission or any other manifestation of a regular magnetic eld. Thus, while the inferred eld strengths are comparable to those found in spiral galaxies, the coherence scale for these elds is much smaller than the scale of the galaxy itself. Recently, magnetic elds were observed in the dwarf irregular galaxy NGC 4449. The mass of this galaxy is an order of magnitude lower than that of the typical spiral and shows only weak signs of global rotation. Nevertheless, the regular magnetic eld is measured to be 6 8 µ G, comparable to that found in spirals (Chyzy et al. 2000). Large domains of non-zero Faraday rotation indicate that the regular eld is coherent on the scale of the galaxy. This eld appears to be composed of two distinct components. First, there is a magnetized ring 2.2 kpc in radius in which clear evidence for a regular spiral magnetic eld is found. This structure is reminiscent of the one found in M31. Second, there are radial “fans” – coherent magnetic structures that extend outward from the central star forming region. Both of these components may be explained by dynamo action though the latter may also be due to out ows from the galactic center which can stretch magnetic eld lines.D. Galaxy ClustersGalaxy clusters are the largest non-linear systems in the Universe. X-ray observations indicate that they are lled with a tenuous hot plasma while radio emission and RM data reveal the presence of magnetic elds. Clusters are therefore an ideal laboratory to test theories for the origin of extragalactic magnetic elds (see, for example, Kim, Tribble, & Kronberg 1991; Tribble 1993). Data from the Einstein, ROSAT, Chandra, and XMM-Newton observatories provide a detailed picture of rich galaxy clusters. The intracluster medium is lled with a plasma of temperature T 107 108 K that emits X-rays with energies ～ 1 10 keV. Rich clusters appear to be in approximate hydrostatic equilibrium with virial velocities ～ 1000 km s 1 (see, for example, Sarazin 1986). In some cluster cores, the cooling time for the plasma due to the observed X-ray emission is short relative to the dynamical time. As the gas cools, it is compressed and ows inward under the combined action of gravity and the thermal pressure of the hot outer gas (Fabian, Nulsen, & Canizares 1984). These cooling ows are found in elliptical galaxies and groups as well as clusters. The primary evidence for cooling ows comes from X-ray obse