The wavelength dependence of optical polarization in the galaxy M82

New linear polarization data are presented for the galaxy M82 in red light using the Durham polarimeter and a McMullan electronographic camera. The degree of polarization found was substantially smaller than for previously obtained blue data. In both colours the NW side is less highly polarized than the SE side. These results indicate a NW-SE asymmetry in the polarized intensity together with a dilution by redder unpolarized light. If M82 is tilted at approximately 100 to the edge on position with the nearer half to the NW, scattered light off dust grains to the NW of the disc must pass through the disc dust before reaching the observer. Polarized light from both sides is diluted by unpolarized disc light. The required brightness distribution and optical thickness of the disc appear reasonable, and a fairly constant halo dust density is predicted. If M82 is virtually edge on the NW-SE asymmetry in degree of polarization could still be obtained if there is more dust to the SE. Dilution could result from halo starlight or intrinsic gas emission. However, the latter is unlikely unless gas emission is extremely strong at intermediate distances along the minor axis of the galaxy. (author)