ar X iv : a st ro - p h / 98 05 34 2 v 1 2 8 M ay 1 99 8 Searching for WR stars in I Zw 18 — The origin of He II emission 1

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 λ4686 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 λ4686 emission is associated with the NW stellar cluster, clearly displaced from the surrounding shell-like [O III] and Hα emission. We found evidence for He II sources, compatible with 5–9 WNL stars and/or compact nebular HeII λ4686 emission, as well as residual diffuse emission. Only one of them is outside the NW cluster. We have done an extensive comparison between our results and the recent ground-based data used by Izotov et al. (1997) and Legrand et al. (1997) to identify WN and WC stars in I Zw 18. The differences between the various data may be understood in terms Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., for NASA under contract NAS5-26555 Space Telescope Science Institute, Baltimore, MD 21218