Fast and Slow Density Waves in Magnetized Spiral Galaxies

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Abstract
Nonthermal synchrotron radio emissions from gas-rich disk spiral galaxies reveal the presence of spiral magnetic field structures over galactic scales, besides the luminous optical spiral arms. From polarized radio observations, large-scale magnetic fields are inferred to lie within galactic planes and to orient roughly in the azimuthal direction. Using the magnetohydrodynamic (MHD) approximation, we develop a theory for galactic MHD density waves in a thin magnetized rotating gaseous disk, present analytical calculations for both fast and slow MHD density waves sustained by self-gravity in the tight-winding approximation, and study the phase relationships among velocity, magnetic field, and density perturbations of these MHD density waves. In particular, we relate fast MHD density waves to the global pattern of roughly coincident optical and magnetic spiral structures seen, for example, in the "Whirlpool galaxy" M51 (NGC 5194) and apply slow MHD density waves to the spiral structure in which magnetic spiral arms are interlaced with optical spiral arms discovered recently in the nearby galaxy NGC 6946. Slow MHD density waves might also manifest in other late-type gas-rich spiral galaxies such as the galaxies IC 342 and M83 (NGC 5236).