A simple and accurate approximation for the Q stability parameter in multi-component and realistically thick discs

TL;DR

This paper introduces a new, simple, and accurate Q stability parameter for multi-component, thick galactic discs, enabling better prediction of local stability by accounting for various components and disc thickness effects.

Contribution

The paper proposes a novel Q_N parameter that improves stability analysis by incorporating multiple components and disc thickness, demonstrated on THINGS galaxy data.

Findings

H_2 significantly influences disc stability even at large radii.

HI has negligible impact on stability up to the optical edge.

Most THINGS spirals are stable with Q_N well above unity.

Abstract

In this paper, we propose a Q stability parameter that is more realistic than those commonly used, and is easy to evaluate [see Eq. (19)]. Using our Q_N parameter, you can take into account several stellar and/or gaseous components as well as the stabilizing effect of disc thickness, you can predict which component dominates the local stability level, and you can do all that simply and accurately. To illustrate the strength of Q_N, we analyse the stability of a large sample of spirals from The HI Nearby Galaxy Survey (THINGS), treating stars, HI and H_2 as three distinct components. Our analysis shows that H_2 plays a significant role in disc (in)stability even at distances as large as half the optical radius. This is an important aspect of the problem, which was missed by previous (two-component) analyses of THINGS spirals. We also show that HI plays a negligible role up to the edge of the optical disc; and that the stability level of THINGS spirals is, on average, remarkably flat and well above unity.