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 Bi Conti and Vacca 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 initial mass function (IMF) slopes (alpha similar to 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 that account for stellar winds, we synthesize the nebular and WR He II lambda 4686 emission in young starbursts. For metallicities 1/5Z(circle dot) less than or equal to Z less than or equal to Z(circle dot), 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.