Polarisation detection at radio wavelengths in three spiral galaxies

Polarisation at radio wavelengths has been measured in the disk radiation of M51, M81 and M31 using the Westerbork Synthesis Radio Telescope. The magnetic field of M51, as implied by the polarisation vector map, is oriented preferentially in the tangential direction. The intrinsic polarisation of the disk radiation of M51 is estimated to be, on average, 12 %, corresponding to an anisotropy (or degree of uniformity) in the magnetic field of 14 % with scale length <= 3 kpc. A 7 % lower limit is set on the disk polarised radiation of M81, but on some parts of the disk of M31 it reaches 20 %. (author)