Density for stars and gas for the coupled system and the corresponding term due to the dark matter halo |(dK_z/dz)|/4πG (Eq. (1)) vs. z, at R = 6 kpc (panel a) and at R = 18 kpc (panel b). Because of its lower dispersion, the mid-plane H₂ gas density is comparable to the stellar value, and hence despite its low surface density, it can significantly affect the stellar distribution at R = 6 kpc. In the outer Galaxy, the halo gravitational effect dominates, hence it is mainly responsible for the determination of the vertical stellar distribution.