About the Initial Mass Function and He [CSC]ii[/CSC] Emission in Young Starbursts

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Abstract
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 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 (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 that account for stellar winds, we synthesize the nebular and WR He II λ4686 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.
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