Is the Interstellar Gas of Starburst Galaxies Well Mixed

The extent to which the interstellar medium (ISM) in galaxies is well mixed is not yet settled. Metal abundances measured in the diffuse neutral gas of star-forming gas-rich dwarf galaxies are deficient with respect to those of the ionized gas. The reasons, if real, are not clear and need to be based on firm grounds. Far-UV spectroscopy with FUSE of giant Hii regions, such as NGC 604 in the spiral galaxy M33, allows us to investigate possible systematic errors in the metallicity derivation. We still find underabundances of nitrogen, oxygen, argon, and iron in the neutral phase by a factor of ∼ 6. This could be explained either by the presence of less chemically evolved gas pockets along the lines of sight, or by dense clouds from which Hii regions form. Those could be more metal rich than the diffuse medium.