Importance for high redshift galaxies

Recent spectra of high redshift galaxies, witnessed at a time when the universe was only a few billion years old, bear a striking resemblance to nearby starburst galaxies, which themselves show the characteristic wind signatures of hot, O-type stars. However, we are poorly equiped to interpret these important data, since the properties of their constituent stars remain poorly known.

Solely hydrogen and helium were formed in the Big Bang, so that all heavier elements were subsequently created through nuclear reactions in stars. Consequently, the heavy element content (or metallicity) of a young galaxy will be much lower than that of the current Milky Way. The metallicity of a galaxy plays a crucial role in massive star evolution, since it defines their internal structure, opacities and stellar wind properties. The precise relation between metallicity and mass-loss, a key ingredient for the reliable population synthesis of young galaxies, is poorly known.

The most direct method of deriving empirical mass-loss rates for hot stars is through analysis of the UV resonance transitions from dominant metal ions. However, difficulties in deriving the wind ionization balance using currently available (trace) ions means that mass-loss rates remain uncertain. With FUSE, additional wind lines will be observed, spanning a much wider range of species. From these data, the degree of ionization can be determined accurately, so that mass-loss rates can be measured with confidence. In addition, FUSE will observe massive stars in both the Galaxy and the Magellanic Clouds - spanning a factor of ten in metallicity - so that the variation of mass-loss properties with metal content will be measured, of importance in the study of high redshift galaxies.