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4.2. Regeneration and amplification of magnetic fields in the intercluster medium

Early ideas on the amplification of magnetic fields in galaxy clusters came from attempts to explain what is probably a closely coupled phenomenon - namely the re-acceleration of ICM relativistic electrons. This is required by observations which confirm their spatial distribution well away from active galaxies and radio sources near the cluster center - for example in the Coma cluster. Jaffe (1980) proposed that turbulent wakes behind galaxies moving through the ICM would re-accelerate relativistic electrons, and maintain the magnetic field at µG levels. Similar ideas were discussed by Roland et al (1981), and Ruzmaikin et al (1989) who suggested a turbulent dynamo mechanism. More recently Goldman and Rephaeli (1991), De Young (1992) and Tribble (l993), proposed that cluster mergers provide the conditions required to explain the radio synchrotron halos in a small minority of clusters (like Coma). They argue, in contrast, that turbulence due to galactic wakes are not effective enough to amplify ICM fields to µG levels, as is observed.

An important element is to understand the physics of cooling flows (cf Edge et al 1992), since there is strong circumstantial evidence (section 4.1.3) that the generation of very strong magnetic fields is connected with the cooling flow process. Recent results described above indicate that B2 / 8pi is, at least locally, energetically comparable to the turbulent and thermal energy densities, and possibly more energetic than the dynamical energy density of the cooling flow itself. However, > 1 µG fields also exist in some clusters which are not of the `cooling flow' type, and for both these classes of cluster, substantial magnetic fields are also found well beyond the cluster core. Soker and Sarazin (1990) propose how a large scale, radially oriented field structure would be produced by a cooling flow. It is not clear, however, that the recently discovered strong RM features are consistent with this model. The reduced conductivity in cooling flow clusters naturally creates conditions for multiphase cooling flows, as recently suggested, and modeled by Tribble (1992). On the contrary, Rosner and Tucker (1989) argue that tangled intracluster magnetic fields do not significantly reduce the thermal conduction in clusters, which leads them to suggest that cooling flow rates in giant cluster-dominating elliptical galaxies are typically leq 0.1 Modot / yr, which is up to an order of magnitude smaller than has generally been thought.

It has been suggested (e.g. Ruzmaikin et a1 1989, Eilek 1993) that, at least for cooling flow clusters, large scale dynamo amplification is operating to produce such strong fields. Our understanding of the details will improve with further radio and especially high resolution multi-band x-ray images in future. Given the complex mix of gravitational and (now evidently) magnetic forces at play, the problem to solve has a highly non-linear character, and will probably require numerical solutions by analogy with the MHD equations which have recently provided insight into the physics of radio source jets and lobes. Fast dynamo amplification of magnetic fields in radio lobes (figure 11 and section 3.2.3) may indicate an analogous process for magnetic field amplification in cooling flows.

With recent evidence for µG-level fields in most clusters, some form of dynamo is probably also responsible, although at what stage in the cluster's history is not yet clear. The answer to this question will illuminate the role of magnetic forces in galaxy evolution in general. If massive inflow on a cluster scale can amplify magnetic fields, the process may also work effectively for infalling gas onto a single galaxy, as Pudritz and Silk (1989) and Pudritz (1990) have suggested to explain galactic magnetic field strengths.

The foregoing results and ideas reveal an emerging picture of extragalactic magnetic fields which was not available to us a few years ago: magnetic field strengths in galaxy clusters (excepting dense cooling flow filaments, and the lobes of extragalactic radio sources) appear to be of order 1 to a few microgauss. This is comparable with that found in the interstellar medium of our own galaxy, where the co-spatial gas density is much higher. Both the statistical evidence for Abell clusters (Kim et a1 1991), and the supracluster emission found around the Coma cluster (see next section) suggest that microgauss-level fields extend beyond the core regions of clusters, where the gas density is lower still. Intergalactic magnetic field strengths appear to care little about the density of their associated thermal gas; what this seems to be telling us is that there is a near-universal interstellar/intergalactic field strength in the local universe, whose energy density is close to that of the microwave background radiation, varepsilonbg. The background-equivalent magnetic field strength Bbge approx 3 × 10-6 G. If there is a physical cause-effect connection to varepsilonbg, e.g. via the cosmic ray background energy density (cf section 5.4.4), which is also similar, this would provide at least empirical support for the idea that interstellar and intergalactic magnetic fields have always `saturated' to Bbge, barring other localized processes which may drive the field higher. Examples of the latter would be dense cooling flows in clusters, radio jets and lobes, and very dense, star-forming molecular clouds. It seems reasonable, and not inconsistent with observation, to postulate that galaxy systems have evolved in a magnetic environment where |B| gtapprox 1 µG over most of the cosmic look-back time.

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