Magnetohydrodynamic Density Waves in a Composite Disk System of Interstellar Medium and Cosmic-Ray Gas

Multi-wavelength observations from radio to soft X-ray bands of large-scale galactic spiral structures offer synthesized and comprehensive views of nearby disk galaxies. In the presence of a massive dark-matter halo, the density-wave dynamics on galactic scales involves the stellar disk, the gas disk of interstellar medium (ISM), the magnetic field, and the cosmic-ray gas (CRG). In this paper, we explore the dynamic and electromagnetic interplay between the magnetized ISM disk and CRG disk so that structural and diagnostic features of optical, infrared, and synchrotron radio-continuum emissions from a spiral galaxy can be physically understood. On timescales of galactic density waves, cosmic rays collectively may be treated as a relativistically hot tenuous gas fluid that is tied to the large-scale mean magnetic field in transverse bulk motions but moves otherwise differently along the magnetic field relative to the ISM. For both fast and slow magnetohydrodynamic (MHD) density waves in a composite disk system of magnetized ISM and CRG, the minute CRG mass density enhancement is phase shifted relative to the enhancement of parallel magnetic field. Owing to an extremely small number of cosmic rays, the large-scale magnetic field enhancement dominates in synchrotron radio-continuum emissions (as if the CRG is almost unperturbed) for spiral structural manifestations. In addition to the fast and slow MHD density waves, there also exists a suprathermal MHD wave mode by which CRG adjusts itself with an effective suprathermal sound speed close to the speed of light c.