Simulations Of Mergers With Magnetic Fields

Faraday rotation measurements indicate that clusters are permeated by magnetic fields of the order of 1/xG (e.g. Kim et al. 1991). Also radio halos require the existence of magnetic fields in clusters on scales of a few Mpc (e.g. Giovannini et al. 1991, 1993). Therefore magneto-hydrodynamic calculations have been performed (Dolag et al. 1999; Roettiger et al. 1999b) to investigate the origin, distribution, strength and evolution of the magnetic fields (see Fig. 8.6). The results of these simulations show that the initial field distribution at the beginning of the simulations at high redshift is irrelevant for the final structure of the magnetic field. The final structure is dominated only by the cluster collapse. Faraday rotation measurements can be reproduced by the simulations for magnetic fields of the order of 1/jG, in very good agreement

Cluster Xyz

Figure 8.6. Isodensity surface of a cluster model with a central magnetic field of 1.1 nG at redshift z=0. The three projections show the rotation measure (colour) and the gas density (contours). The purple arrows indicate the orientation and the strength of the magnetic field in a plane containing the cluster centre. The inset shows the rotation measure versus distance from the cluster centre plotted using various simulations (lines) and observations in several clusters (symbols) (from Dolag 2001).

Figure 8.6. Isodensity surface of a cluster model with a central magnetic field of 1.1 nG at redshift z=0. The three projections show the rotation measure (colour) and the gas density (contours). The purple arrows indicate the orientation and the strength of the magnetic field in a plane containing the cluster centre. The inset shows the rotation measure versus distance from the cluster centre plotted using various simulations (lines) and observations in several clusters (symbols) (from Dolag 2001).

with the value inferred from observations. The models reproduce very well also the radial decline of the magnetic field found in the observations (Clarke et al. 2001, Dolag et al. 2001).

Shear flows are extremely important for the amplification of the magnetic field, while the compression of the gas is of minor importance. Mergers change the local magnetic field strength drastically. But also the structure of the cluster-wide field is influenced. At early stages of the merger the filamentary structures prevail. This structure breaks down later (~ 2-3 Gyr) and leaves a stochastically ordered magnetic field.

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