Upper panel: Vertical density profiles (solid curves) of gas and stars of a two-component isothermal disc with μ = 9 and ξ = 0.3. The dashed curves are the profiles of the corresponding SGI models with the same velocity dispersion and surface density. Lower panel: Vertical Q_3D profiles of the same models as in the upper panel, assuming Q_gas = 0.6 (and thus Q_⋆ = 1.4). The horizontal dashed line indicates the instability threshold, Q_3D = 1. Here, ρ_0, gas and b_gas are, respectively, the midplane density and the scale height of the gaseous component of the two-component disc.