He II emission in extragalactic H II regions

Astronomy and Astrophysics – Astrophysics

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Astronomical Spectroscopy, H Ii Regions, Helium Ions, Light Emission, Magellanic Clouds, Dwarf Galaxies, Line Spectra, Nebulae, Photoionization, Star Formation

Scientific paper

Spectroscopic observations confirming the presence of nebular He II 4686-A emission in the SMC H II region N76 are presented, and it is demonstrated that the He II emission associated with the WO star in IC 1613 is also extended. The properties of four H II regions are discussed. The close correlation of the emission with specific stars indicates that photoionization by the stars themselves is the excitation mechanism, and it is proposed that this may be true for those nebulae ionized by WO stars and some rare high-excitation WN stars. The existence of these nebulae with He II emission increases the likelihood that the 4686-A emission frequently observed in dwarf emission-line galaxies is nebular in origin. This prompts the conclusion that the radiation field associated with star-forming regions can be harder than previously suspected, and reopens the issue of whether photoionization by stars in young galaxies can account for the ionization observed in QSO absorption-line systems.

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