Greyscale plot of the observed intensity of polarized emission from M 101 at λ 6.2 cm and apparent B-vectors (E+90°, not corrected for Faraday rotation) with length proportional to the degree of polarization. A vector of 1′  length corresponds to 20%. The noise level in PI is 0.07 mJy/beam area . Contours show the brightness distribution of H i   of Braun (1995). The contour levels of column density are (10, 15, 20, and 25) 10²⁰ cm ^-2. The beamwidth of is shown in the lower right corner.