  A new solution is presented for the puzzling, observed universality of the
exponential luminosity profiles, perpendicular to the disk plane of spiral and
lenticular galaxies. It is shown that such exponential $z$-profiles result
naturally from gaseous protodisks which settle into isothermal equilibrium
prior to star formation. Subsequent cooling leads to a gravitational
contraction of the gas towards the equatorial plane and to a stellar
exponential $z$-profile if the star formation rate is assumed to be comparable
to the cooling rate of the gas. The final stellar scale height depends only on
the initial gas temperature and local surface density. This model therefore
provides a new method to investigate the early energetic state of galactic
protodisks with measured scale heights and surface densities along the disk
plane.
