1998A&A...335..123F

Wide-field images of NGC 6946 in linear optical polarization are presented. In large parts of the galaxy the polarization pattern delineates a spiral structure, in partial agreement with the radio polarization data. A possible explanation of such a pattern is the alignment of elongated dust grains in the regular interstellar magnetic field. The optical degree of polarization anti-correlates with the stellar density: the bright parts of the spiral arms and the nuclear region reveal the lowest degrees of polarization (below 0.5%), while the outer, less active regions are significantly polarized (up to 5%). This trend is similar as in the degree of polarization of radio synchrotron emission. This behavior could be interpreted as field tangling in star-forming regions. The higher resolution of our optical data would then allow to determine a new upper limit of the turbulence scale of ≃120pc. In the southern and southwestern parts of NGC 6946 the polarization angles deviate from a spiral structure, indicating the exceptional character of these regions as known from observations in other frequency bands. However, we cannot exclude that here the optical polarization structure is influenced by effects external to the galaxy. The southern region of strong optical polarization and constant vector orientation might be interpreted as enhanced foreground polarization in a Galactic gas/dust cloud.