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

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

1998-12-18

arxiv.org/abs/astro-ph/9812345v1

Astronomy and Astrophysics

Astrophysics

To appear in "Dwarf Galaxies: Probes for Galaxy Formation and Evolution'', Ed. J. Andersen, Highlights of Astronomy, Vol. 11,

Scientific paper

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.

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