SIMBAD references

We present radio continuum measurements of the edge-on spiral galaxy NGC4631 at 4.86 and 8.44GHz. We discuss the morphology of the radio halo in total and polarized emission and its wavelength dependence. The rotation measure and intrinsic magnetic field orientation are determined using the two frequency pairs 4.86/8.44GHz and 1.49/4.86GHz, the latter with the help of published 1.49GHz data. The results can be summarized as follows: i) The rotation measure in the radio halo is between -15 and +20rad/m2. The intrinsic large-scale B-field in the halo is quite symmetric with respect to the major and minor axes of NGC4631. It generally points to, or away from the central 5kpc. ii) The prominent radio spurs of NGC4631 are found to be an essential constituent of the radio halo. In all the regions where the B-field structure can be determined, the field orientation follows the spurs. iii) The sources of the cosmic rays in the radio halo are most probably located in the disk of NGC4631 and associated with star-forming regions. In particular the NE-spur seems to have several tributaries, each originating at an HII region complex. iv) The magnetic field structure is essential in explaining the unusual size and structure of the radio halo. The field in the halo probably results from an outflow of gas from the disk into the halo, which is caused or promoted by the gravitational interaction with NGC4656. Based on the available observations and dynamo models, the existence of an α²- or αω-dynamo as the source of, or as a contributor to, the halo magnetic field can neither be proven nor disproven. v) The depolarization in the plane of NGC4631 has a clear wavelength dependence in the sense that there is less depolarization at higher frequencies. Faraday depolarization is therefore an essential process in NGC4631.