ISO observations of starburst and ultraluminous galaxies

We report first results on Infrared Space Observatory (ISO) spectroscopic observations of Starburst and Ultraluminous Galaxies. Observations of a wide range of ionic, atomic, and molecular infrared lines enable us to explore in detail the physical conditions in the circumnuclear regions of these galaxies, and to determine, as a first step, parameters such as gas densities and temperatures of ionizing stars. With the help of theoretical modeling this multi-line spectroscopic database is used to further probe the central emission mechanism, discriminate between stellar and non-thermal processes and finally constrain the current models of galactic nuclei.