Stellar and gas (HI) vertical density distribution, ρ(z) vs. z at R = 8.5 kpc (panels a and b, respectively). In each plot, the solid curve represents the vertical distribution in the one-component case. The dashed curve represents the distribution obtained in the gravitationally coupled stars-plus-gas system. The coupled gravitational force of the star- plus-gas system constrains each distribution towards the mid-plane, i.e., increases the mid-plane density value of the distribution, which causes the scale height to become smaller and the curve to become steeper.