Integrand −zρ(z)K_z vs. z (from Eqs. (2), (13)) of W (potential energy per unit area) for stars and gas (HI) at R = 8.5 kpc. Panela: energy integrand for stars in the stars-alone case (solid curve) and in the coupled stars-plus-gas case (dashed curve). Twice the area under the curves gives the energy per unit area of the stellar disk in the two cases, which is obtained to be the same. Panel b: energy integrand for gas (HI) in the gas-alone case (solid curve) and in the coupled stars-plus-gas case (dashed curve). Twice the area under the curves gives the energy per unit area of the gas disk in the two cases, which are obtained to be the same.