FAST AND SLOW DENSITY WAVES IN MAGNETIZED SPIRAL GALAXIES

Nonthermal synchrotron radio emissions from gas-rich disk spiral galax ies reveal the presence of spiral magnetic field structures over galac tic scales, besides the luminous optical spiral arms. From polarized r adio observations, large-scale magnetic fields are inferred to lie wit hin galactic planes and to orient roughly in the azimuthal direction. Using the magnetohydrodynamic (MI-ID) approximation, we develop a theo ry for galactic MHD density waves in a thin magnetized rotating gaseou s disk, present analytical calculations for both fast and slow MHD den sity waves sustained by self-gravity in the tight-winding approximatio n, and study the phase relationships among velocity, magnetic field, a nd density perturbations of these MI-ID density waves. In particular, we relate fast MHD density waves to the global pattern of roughly coin cident optical and magnetic spiral structures seen, for example, in th e ''Whirlpool galaxy'' M51 (NGC 5194) and apply slow MI-ID density wav es to the spiral structure in which magnetic spiral arms are interlace d with optical spiral arms discovered recently in the nearby galaxy NG C 6946. Slow MI-ID density waves might also manifest in other late-typ e gas-rich spiral galaxies such as the galaxies IC 342 and M83 (NGC 52 36).