  Over the past two decades observations have revealed that the vertical
density distribution of stars in galaxies near the mid-plane is substantially
steeper than the sech^2 function that is expected from an isothermal
approximation. However, the physical origin for this has not been explained so
far. Here we show that such steep profiles result naturally even within the
isothermal regime, on taking into account the gravitational force due to the
gas. Due to its low velocity dispersion the gas is concentrated closer to the
galactic mid-plane than the stars, and hence it strongly affects the vertical
stellar distribution even though its contribution to the total surface density
is small. We apply a three-component galactic disk model consisting of
gravitationally coupled stars and the HI and H_2 gas, embedded in the dark
matter halo, and calculate the vertical density distribution of stars for the
Galaxy. The resulting vertical density distribution of stars is shown to be
steeper than the sech^2 function, and lies between the sech and an exponential
function, in good agreement with observations of galaxies. We also show that a
multi-component stellar disk consisting of coupled dwarfs and two populations
of giants does not explain the observed steep stellar profiles.
