The nature of the magnetic belt in M 31

The most conspicuous feature of the large-scale magnetic field in the Andromeda nebula (M31) is a magnetic belt centred at a galacto-centric radius of about 10 kpc. We suggest a nonlinear dynamo model for M31, which associates this belt with the gas density profile as well as with the kinematic parameters of M31. Our model is based on the observed density profile and vertical structure of the gas disc of M31, and on a recent rotation curve. The traditional explanation of the belt in kinematic dynamo theory was connected with a double-peaked form of the rotation curve of M31. We show that this explanation fails with recent determinations of the rotation curve of M31. Furthermore, it is unsatisfactory because it underestimates, even in the kinematic (linear) regime, the interplay of magnetic fields generated at different galacto-centric radii. Our model predicts an extended belt of magnetic field between 7 and 12 kpc radius, in accordance with observations. However, our models typically also have another maximum of the regular magnetic field between 2 and 6 kpc radius which is not obvious in the synchrotron emission. We discuss possible reasons why it could avoid detection if it is real, and suggest a search for the field in this region, e.g. via Faraday rotation of polarized radio sources behind M31. A further generic feature of our models is that the scale height of the regular magnetic field is significantly larger than that of the gas, and has a steeper increase with galacto-centric radius. This can be important, e.g., for cosmic ray confinement and vertical gas balance.