The magnetic field structure in the radio halos of NGC 891 and NGC 4631.

Linearly polarized radio continuum emission has been detected in the halos of NGC 891 and NGC 4631. In both galaxies the magnetic field shows uniform structures over scales of several kpc. The field lines point radially outward from the disk of NGC 4631, while in NGC 891 no overall orientation is visible. The increase in the degree of polarization with increasing distance from the plane is explained by Faraday depolarization by thermal gas with scale heights of 0.9 and 1.3 kpc for NGC 891 and NGC 4631, respectively. The value for NGC 891 agrees with recent Hα data. The magnetic field is assumed to be in equipartition with the cosmic ray energy density. In that case the scale heights of the fields are ~4 and ~8 kpc, respectively, which is consistent with the depolarization data. Hence the field lines reach far out into the halo. In the outer halo, the degree of polarization decreases again, maybe due to an increasing field turbulence. Both dynamo and galactic wind models can explain the two observed field structures. Either NGC 891 and NGC 4631 have dynamo fields of different modes (quadrupole and dipole), or the galactic wind is much stronger in NGC 4631.