Magnetohydrodynamic density waves in a composite disc system of interstellar medium and cosmic ray gas

Multiwavelength observations from radio to soft X-ray bands of large-scale galactic spiral structures offer synthesized and comprehensive views of nearby disc galaxies. In the presence of a massive dark matter halo, the density-wave dynamics on galactic scales involves the stellar disc, the gas disc of the 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 disc and the CRG disc so that structural and diagnostic features of optical, infrared and synchrotron radio-continuum emissions from a spiral galaxy can be understood physically. On time-scales 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 disc system of magnetized ISM and CRG, the minute CRG mass-density enhancement is phase shifted relative to the enhancement of the parallel magnetic field. Owing to the 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 speed of sound close to the speed of light c.