Searching for Wolf-Rayet stars in I Zw 18: The origin of He II 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 Hubble Space Telescope WFPC2 imaging and Faint Object Spectrograph. spectroscopy, we were able to improve previous studies. We constructed a continuum-free He II lambda 4686 map, which was used to identify Wolf-Rayet (W-R) stars recently found by ground-based spectroscopy and to locate diffuse nebular emission. Most of the He II lambda 4686 emission is associated with the northwest stellar cluster clearly displaced from the surrounding shell-like [O III] and H alpha emission. We found evidence for He II sources, compatible with five to nine WNL stars and/or compact nebular He II lambda 4686 emission, as well as residual diffuse emission. Only one of them is outside the northwest cluster. We have done an extensive comparison between our results and the recent ground-based data used by Izotov et al. and Legrand et al, to identify WN and WC stars in I Zw 18. The differences between the various data may be understood in terms of varying slit locations, continuum fits, and contamination by nebular lines. We have calculated evolutionary tracks for massive stars and synthesis models at the appropriate metallicity (Z approximate to 0.02 Z.). These single-star models predict a mass limit M-WR approximate to 90 M. for W-R stars to become WN and WC/WO. For an instantaneous burst model with a Salpeter initial mass function extending up to M-up approximate to 120-150 M., 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 He II lambda 4686 emission due to the presence of WC/WO stars. This quantitative agreement and the spatial correlation of nebular He II lambda 4686 with the stellar cluster and the position of W-R stars shown from the ground-based spectra further supports the hypothesis that W-R stars are responsible for nebular He II emission in extragalactic H II regions.