The ring-like or extremely tight spiral structures of Andromeda nebula (M31 or NGC 224) revealed in multi-wavelength observations are interpreted in terms of fast magnetohydrodynamic (MHD) density waves in a rotating composite galactic system of a stellar disk and a magnetized gas disk controlled by a massive halo. For such waves inside the Lindblad resonance, perturbation enhancements of stellar and gas surface mass densities as well as azimuthal magnetic field are roughly in phase. Magnetic fields are very important in orchestrating various components of the interstellar medium (ISM) on large scales and affect multiple sub-processes that occur in the ISM on smaller scales. Collectively, this leads to an overall correlation of large-scale annulus structures observed. It is natural that the enhanced magnetic field tends to align along the magnetic torus as revealed by polarization studies. The low star-formation rate of M31 may be related to the stability of the composite MHD system. The scenario of MHD density waves is valid for magnetized spiral galaxies including large-scale annulus structures in galaxies such as NGC 2841 and NGC 7331 etc.
Proceedings 232. WE-Heraeus Seminar
- Pub Date:
- December 2000