Total emission and apparent B-vectors of the polarized emission (defined as E-vectors rotated by 90°) from M 101 observed at λ  6.2 cm overlaid on the optical image of Sandage (1961). The contour levels are 1, 2, 4, 8, 12, 16, and 24  mJy/beam area . A vector of 1′  length corresponds to a polarized intensity of 1 mJy/beam area . The noise levels are 0.5  mJy/beam area in I and 0.07 mJy/beam area in PI. The beamwidth of is shown in the lower right corner. A square-root scale has been applied to the optical image to show low surface brightness structures more clearly.