Polarisation detection at radio wavelengths in three spiral galaxies

Polarization at radio wavelengths has been measured in the disk radiation from M51, M81, and M31 using the Westerbork Synthesis Radio Telescope. The magnetic field of M51, as implied by the polarization-vector map, is oriented preferentially in the tangential direction. The intrinsic polarization of the disk radiation from M51 is estimated to be, on average, 12%, corresponding to an anisotropy (or degree of uniformity) in the magnetic field of 14% with a scale length not exceeding 3 kpc. A 7% lower limit is set on the polarized disk radiation from M81, but in some parts of the disk of M31 it reaches 20%.