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

TL;DR

This study examines how the finite thickness of galactic discs and the presence of interstellar gas influence the stability and longevity of spiral density waves, revealing opposing effects that determine spiral wave persistence.

Contribution

It provides a detailed analysis of the combined effects of disc thickness and interstellar gas on spiral wave stability and lifetime using dispersion relations in a WKB framework.

Findings

Disc thickness reduces self-gravity, stabilizing against perturbations.

Interstellar gas extends the lifetime of spiral density waves.

Opposite effects of gas and thickness determine spiral wave longevity.

Abstract

A typical galactic disc has a finite thickness and 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 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 different thickness for stars and gas). For each case, we derive the appropriate dispersion relation in the WKB approximation, and study the dynamical effect of the disc thickness on the life-time 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. Further, the interstellar gas and the disc thickness are shown to have a mutually opposite dynamical effect on the disc stability as well as 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.