Spiral Magnetohydrodynamic Density Waves with a Tangential Shear Force

Abstract

Evidenced by synchrotron radio-continuum emissions from spiral galaxies, magnetic fields with estimated strengths of several microgauss or stronger are embedded in the interstellar medium (ISM) of spiral galaxies and play important dynamic and diagnostic roles in large-scale galactic density-wave spiral structures as well as in the formation of tight circumnuclear spirals in the central regions where magnetic field strengths may be higher than several tens of microgauss. In this paper, we examine several physical effects of the tangential shear force on spiral magnetohydrodynamic (MHD) density waves in a thin magnetized gas sheet with differential rotation. This tangential shear force in the azimuthal direction, mainly caused by the disk differential rotation, is of inertial type and operates only for nonaxisymmetric MHD density wave perturbations. We focus on shear-related processes of interest: (1) The short-, long-, and open-MHD density waves may become unstable near corotation. (2) Fast magnetoacoustic density waves may become overstable (oscillatory growth) outside the Lindblad resonances. (3) Slow MHD density waves may become unstable near corotation. We discuss relevant theoretical works and conceptual applications of these results to multiwavelength galactic observations, including spiral galaxies, barred spiral galaxies, and tight circumnuclear spirals in central regions of spiral galaxies. In terms of the angular momentum transport, spiral MHD density waves driven by the tangential shear force should also find important applications in magnetized accretion disks in broader astrophysical contexts.