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Astron. Astrophys. 335, 1117-1123 (1998)

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New clues to the magnetic field structure of M 31

J.L. Han 1, 2, R. Beck 1 and E.M. Berkhuijsen 1

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
2 Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, P.R. China (hjl@class1.bao.ac.cn)

Received 27 January 1998 / Accepted 21 April 1998


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 [FORMULA]6 cm and [FORMULA]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.

Key words: Galaxies: magnetic fields – - Galaxies: halos – - Galaxies: ISM – - Galaxies: individual: M 31 – - Polarization

Send offprint requests to: J.L. Han

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998