2000MNRAS.315..646L

The Andromeda galaxy (also referred to as M31 or NGC 224) shows approximately axisymmetric large-scale structures which are identified here with fast magnetohydrodynamic (MHD) density waves in a simple theoretical model characterized by a rotating composite system of a massive spherical halo (dark matter included), a stellar disc and a magnetized gas disc. For fast MHD density waves inside the Lindblad resonance, enhancements of stellar and gas surface mass densities as well as the azimuthal magnetic field are all roughly in phase. Based on this analysis and considerations of radiation processes in a magnetized interstellar medium, the observed overall correlation of annulus structures of M31 at many electromagnetic wavelengths can be naturally explained. In particular, the enhanced magnetic field (inferred from the synchrotron radio continuum) tends to align roughly along the magnetic torus of M31 as revealed by radio polarization studies. From observational estimates of physical parameters, the composite system of M31 is found to be quite stable - a fact that may account for the observed low global star formation rate in M31.