Massive star populations in I Zw 18: A probe of stellar evolution in the early universe

We present a study of the gaseous and stellar emission in I Zw18, the most metal-poor star-forming galaxy known. Archival HST WFPC2 and FOS data have been used to analyze the spatial distribution of [OIII], Halpha, and HeII 4686. The latter is used to identify Wolf-Rayet stars found by ground-based spectroscopy and to locate nebular HeII emission. Most of the HeII emission is associated with the NW stellar cluster, displaced from the surrounding shell-like [OIII] and Halpha emission. We found evidence for HeII sources compatible with 5-9 WNL stars and/or compact nebular HeII emission as well as residual diffuse emission. New evolutionary tracks and synthesis models at the appropriate metallicity predict a mass limit M_WR ~90 Msun for WR stars to become WN and WC/WO. The observed equivalent widths of the WR lines are in good agreement with an instantaneous burst model with a Salpeter IMF extending up to M_up ~ 120-150 Msun. Our model is also able to fully reproduce the observed equivalent widths of nebular HeII emission due to the presence of WC/WO stars. This finding together with the spatial distribution of nebular HeII further supports the hypothesis that WR stars are responsible for nebular HeII emission in extra-galactic HII regions. Finally we discuss the implications on stellar mass loss, chemical yields, final stellar masses, and the ionizing flux of starburst galaxies at very low metallicities.