Annu. Rev. Astron. Astrophys. 1996. 34: 155-206 Copyright © 1996 by Annual Reviews. All rights reserved

3.2. M31: The Nearly Perfect Magnetic Torus

Radio observations of M31 reveal a 20 kpc-diameter torus with the regular magnetic field, as determined from a Faraday rotation model, aligned uniformly with respect to the circumference (Sofue & Takano 1981, Beck 1982, Ruzmaikin et al 1990). A superposition of helical field loops (as expected from dynamo theory) can explain the asymmetric distribution of polarized emission with respect to the minor axis (Donner & Brandenburg 1990, Urbanik et al 1994). The mean equipartition field strength in the torus is 7 µG for the total field and 4 µG for the regular field.

M31 has a low star-formation rate globally and no grand-design spiral structure. Strong compression of magnetic fields by density waves seems to be absent. However, there are many small dust lanes and cloud complexes, which are traced by field lines. Comparison between the total radio intensity and the gas emission in CO and HI confirms a close coupling of the field to the gas clouds Berkhuijsen et al 1993.