COUPLED GALACTIC DENSITY-WAVE MODES IN A COMPOSITE SYSTEM OF THIN STELLAR AND GASEOUS DISCS

A rotating disc galaxy is modelled as a composite system consisting of thin stellar and gaseous discs, which are described by a two-fluid mo dal formalism. The composite disc system is assumed to retain axisymme try in the background equilibrium. General density-wave perturbations in the two discs are coupled through the mutual gravitational interact ion. We study the basic properties of open and tight spiral density-wa ve modes in such a composite disc system. Within the Lindblad resonanc es, perturbation enhancements of surface mass density in stellar and g aseous discs are in phase; this is also true during the initial growth phase of density-wave perturbations. Outside the Lindblad resonances, there exists a possible spiral density-wave branch for which perturba tion enhancements of surface mass density in stellar and gaseous discs are out of phase. We discuss implications of these results on the cri tical parameters for global star formation in barred and normal spiral galaxies and on magnetohydrodynamic density waves within the Lindblad resonances.