New light on the origin of nebular HeII emission in young starbursts

We review the observations of nebular HeII 4686 emission in Local Group objects and extra-galactic HII regions. Different ionization mechanisms proposed to explain the unusually high excitation are summarized. The models of Schaerer (1996) and Schaerer & Vacca (1998) predict nebular HeII emission due to hot WN and WC/WO stars, in good agreement with observations of Wolf-Rayet (WR) galaxies showing WR signatures and/or nebular HeII. Comparisons of synthesis models with observations suggest that nebular HeII emission may be related to WR stars in all cases. The case of I Zw 18 is discussed. It is shown that the observed WR lines and nebular HeII can be reproduced with an instantaneous burst model and a Salpeter IMF. This supports the suggestion that WR stars are responsible for nebular HeII emission.