Interstellar polarization at high galactic latitudes from distant stars

Aims. Interstellar polarization of starlight at high galactic latitudes gives information on the direction of the local Galactic magnetic field and the distribution of cosmic dust in wide “windows” perpendicular to the Galactic plane. Polarization data allow us to construct for the first time high-latitude polarization maps with resolution and sky coverage high enough to examine in detail the distribution of the interstellar polarization and the direction of the Galactic magnetic field around Galactic poles. Methods. We measured the polarization for more than 2400 new stars at distances of up to 600 pc and within 60° and 30° from the north and south Galactic poles. Here we describe the measurements and properties of the resulting total sample of about 3600 stars (completeness, radial distribution), and present polarization maps of the regions around the north (b > 30°) and south (b > 60°) poles. Results. The new interstellar polarization maps give wider and higher resolution views around the Galactic poles than previous maps. The major patterns in the maps are significant asymmetries in the polarization, one in the northern sky directly across the local spiral and the second between the northern and southern Galactic hemispheres. We confirm that there is significantly more interstellar polarization at high southern latitudes than at high northern latitudes within the local spiral. A comparison of our optical polarization map with the proposed models using local dust- and gas shells, (i.e., Weaver’s Loop I superbubble, Wolleben’s two-bubble model, the interaction between the Local Bubble and the Loop I superbubble) reveals interesting features. 1) The optical and radio polarizations are lower in the eastern than in the western branch of Loop I, which may be caused by a weaker effective magnetic field in the eastern part where the diffuse IRAS emission is bright; this illustrates that low optical polarization does not always imply little dust. 2) We can see a clear signature of the western side of Wolleben’s S2 shell (at 35 < l < 55), while there is no alignment of the polarization directions along the suggested wall of S2 in the eastern part, though the alignment along S1 is visible. 3) In the upper parts of the suggested interaction zone between the Local Bubble and the Loop I superbubble our data show polarizations aligned along the zone contour. The week and rather randomly directed polarizations measured previously in the eastern part of this region may reflect the shorter distances (