Searching for WR stars in I Zw 18: the origin of HeII emission

I Zw 18 is the most metal poor star-forming galaxy known and is an ideal laboratory to probe stellar evolution theory at low metallicities. Using archival HST WFPC2 imaging and FOS spectroscopy we were able to improve previous studies. We constructed a continuum free HeII map, which was used to identify Wolf-Rayet (WR) stars recently found by ground-based spectroscopy and to locate diffuse nebular emission. Most of the HeII emission is associated with the NW stellar cluster, clearly 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. Only one of them is outside the NW cluster. We have calculated evolutionary tracks for massive stars and synthesis models at the appropriate metallicity (Z ~ 0.02 Zsun). These single star models predict a mass limit M_WR ~ 90 Msun for WR stars to become WN and WC/WO. For an instantaneous burst model with a Salpeter IMF extending up to M_up ~ 120-150 Msun our model predictions are in reasonable agreement with the observed equivalent widths. 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 quantitative agreement and the spatial correlation of nebular HeII with the stellar cluster and the position of WR stars shown from the ground-based spectra further supports the hypothesis that WR stars are responsible for nebular HeII emission in extra-galactic HII regions. (Abridged abstract