Searching for Wolf‐Rayet Stars in I Zw 18: the Origin of HeiiEmission

I Zw 18 is the most metal poor star-forming galaxy known and is an ideallaboratory to probe stellar evolution theory at low metallicities. Usingarchival HST WFPC2 imaging and FOS spectroscopy we were able to improveprevious studies. We constructed a continuum free HeII map, which was used toidentify Wolf-Rayet (WR) stars recently found by ground-based spectroscopy andto locate diffuse nebular emission. Most of the HeII emission is associatedwith the NW stellar cluster, clearly displaced from the surrounding shell-like[OIII] and Halpha emission. We found evidence for HeII sources, compatible with5--9 WNL stars and/or compact nebular HeII emission, as well as residualdiffuse emission. Only one of them is outside the NW cluster. We havecalculated evolutionary tracks for massive stars and synthesis models at theappropriate metallicity (Z ~ 0.02 Zsun). These single star models predict amass limit M_WR ~ 90 Msun for WR stars to become WN and WC/WO. For aninstantaneous burst model with a Salpeter IMF extending up to M_up ~ 120-150Msun our model predictions are in reasonable agreement with the observedequivalent widths. Our model is also able to fully reproduce the observedequivalent widths of nebular HeII emission due to the presence of WC/WO stars.This quantitative agreement and the spatial correlation of nebular HeII withthe stellar cluster and the position of WR stars shown from the ground-basedspectra further supports the hypothesis that WR stars are responsible fornebular HeII emission in extra-galactic HII regions. (Abridged abstract)