New clues to the magnetic field structure of M 31

We observed 21 polarized background radio sources in the field of M 31 at 1.365 GHz and 1.652 GHz, and determined their rotation measures (RMs). The RM data show that the regular magnetic field of the disk probably extends from about 5 kpc to 25 kpc from the center with similar structure. The RMs obtained from the polarized emission from M 31 at lambda 6 cm and lambda 11 cm indicate that M 31 might have a weak poloidal field in its tenuous halo. Observational features of the odd and even dynamo modes in a galaxy are discussed. An even mode (S0) dynamo may operate in M 31.