About the initial mass function and He II emission in young starbursts

We demonstrate that it is crucial to account for the evolution of the starburst population in order to derive reliable numbers of O stars from integrated spectra for burst ages t > 2−3 Myr. In these cases the method of Vacca & Conti (1992) and Vacca (1994) systematically underestimates the number of O stars. Therefore the current WR/O number ratios in Wolf–Rayet (WR) galaxies are overestimated. This questions recent claims about flat IMF slopes (α ∼ 1–2) in these objects. If the evolution of the burst is properly treated we find that the observations are indeed compatible with a Salpeter IMF, in agreement with earlier studies. Including recent predictions from non–LTE, line blanketed model atmospheres which account for stellar winds, we synthesize the nebular and WR He II λ 4686 emission in young starbursts. For metallicities 1/5Z⊙ ≤ Z ≤ Z⊙ we predict a strong nebular He II emission due to a significant fraction of WC stars in early WR phases of the burst. For other metallicities broad WR emission will always dominate the He II emission. Our predictions of the nebular He II intensity agree well with the observations in WR galaxies and an important fraction of the giant H II regions where nebular He II is detected. We propose further observational tests of our result. Subject headings: galaxies: starburst — H II regions — stars: Wolf-Rayet Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Geneva Observatory, CH-1290 Sauverny, Switzerland Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France