Is the Interstellar Gas of Starburst Galaxies Well Mixed?

Abstract

The extent to which the interstellar medium (ISM) in galaxies is wellmixed is not yet settled. Metal abundances measured in the diffuseneutral gas of star-forming gas-rich dwarf galaxies are deficient withrespect to those of the ionized gas. The reasons, if real, are not clearand need to be based on firm grounds. Far-UV spectroscopy with FUSE ofgiant H II regions, such as NGC 604 in the spiral galaxy M33, allows usto investigate possible systematic errors in the metallicity derivation.We still find underabundances of nitrogen, oxygen, argon, and iron inthe neutral phase by a factor of {\tilde} 6. This could be explainedeither by the presence of less chemically evolved gas pockets along thelines of sight, or by dense clouds from which H II regions form. Thosecould be more metal rich than the diffuse medium.