Mass Depletion from the Nucleus of a Disk Galaxy and its Effect on the Outer Structure of the Galaxy
High-energy explosions in the nuclei of galaxies throw out large amount of gas with sufficiently large velocities. This high-velocity ejections of gas is observed in Seyfert galaxies and in many normal spiral galaxies. The ejected gas flows continuously in the outer region and builds the outer structure of the galaxy. The degree of influence of the outflowing gas on the outer structure depends on the energy released during the explosion and on the damping rate of flow. These two parameters may essentially determine the type of galaxy to be evolved to including the Hubble-type. The physical interpretation of some such observed phenomena has been achieved on the basis of the solution of gas dynamical equations with viscosity.