Dynamical interplay of disc thickness and interstellar gas: Implication for the longevity of spiral density waves

Abstract

A typical galactic disc has a finite thickness. In addition to stars, it also contains a finite amount of interstellar gas. Here, we investigate the physical impact of the finite thickness of a galactic disc on the disc stability against the non-axisymmetric perturbations and on the longevity of the spiral density waves, with and without the presence of gas. The longevity is quantified via the group velocity of density wavepackets. The galactic disc is first modelled as a collisionless stellar disc with finite height and then more realistically as a gravitationally coupled stars plus gas system (with a different thickness for stars and gas). For each case, we derive the appropriate dispersion relation in the Wentzel-Kramers-Brillouin approximation and study the dynamical effect of the disc thickness on the lifetime of spiral density waves via a parametric approach. We find the generic trend that the effective reduction in disc self-gravity due to disc thickness makes it more stable against the non-axisymmetric perturbations and shortens the life-span of the spiral density waves. Furthermore, interstellar gas and disc thickness are shown to have a mutually opposite dynamical effect on the disc stability as well as on the longevity of the spiral density waves. While the gas supports the non-axisymmetric features for a longer time, the disc thickness has an opposite, quenching effect. Consequently, the net change is set by the relative dominance of the opposite effects of the interstellar gas and the disc thickness.