LARGE-FIELD OPTICAL POLARIMETRY OF NGC-891, NGC-5907 AND NGC-7331 - SELECTING THE INTRINSIC POLARIZING MECHANISM

Large-scale images in linear optical polarisation of three edge-on spi ral galaxies (NGC 891, 5907, 7331) are presented. The dominant orienta tion of the polarisation pattern in all observed galaxies is perpendic ular to the major axis and seems to be given by anisotropic scattering effects. However, we also found evidence for polarisation due to the Davis-Greenstein mechanism. In a region located towards the central pa rt of NGC 891 within the dust lane, the derived orientation of the mag netic field is perpendicular to the major axis. In NGC 7331, the polar isation along the western dust lane indicates a toroidal field. No oth er indication for disk-parallel (toroidal) magnetic fields in the obse rved galaxies could be found.