The large scale magnetic field structure of the spiral galaxy NGC 5775

Context. The origin of large-scale magnetic fields in spiral galaxies is still a theoretical riddle and better observational constraints are required to make further progress. Aims. In order to better determine the large-scale 3D-structure of magnetic fields in spiral galaxies we present a Faraday rotation analysis of the edge-on spiral galaxy NGC 5775. Methods. Deep radio-continuum observations in total power and linear polarization were performed at 8.46 GHz with the VLA and the 100-m Effelsberg telescope. They were analyzed together with archival 4.86 and 1.49 GHz VLA-data. We thus can derive rotation measures from a comparison of three frequencies and determine the intrinsic magnetic field structure. Results. A very extended halo is detected in NGC 5775, with magnetic field lines forming an X-shaped structure. Close to the galactic disk the magnetic field is plane-parallel. The scaleheights of the radio emission esimated for NGC 5775 are comaprable with other galaxies. The rotation measure distribution varies smoothly on both sides along the major axis from positive to negative values. Conclusions. From the derived distribution of rotation measures and the plane-parallel intrinsic magnetic field orientation along the galactic midplane we conclude that NGC 5775 has an even axisymmetric large-scale magnetic field configuration in the disk as generated by an alpha Omega-dynamo which is accompanied by a quadrupolar poloidal field. The magnetic field lines of the plane-parallel component are pointing outwards. The observed X-shaped halo magnetic field, however, cannot be explained by the action of the disk's mean-field dynamo alone. It is probably due to the influence of the galactic wind together with the dynamo action.