HE-II EMISSION IN EXTRAGALACTIC H-II REGIONS

We present spectroscopic observations which confirm the presence of nebular He II lambda-4686 emission in the SMC H II region N76 and demonstrate that the He II emission associated with the WO star in IC 1613 is also extended. Several H II regions are now known to show such emission. We discuss the properties of four: N44C and N159F in the LMC, N76, and IC 1613 No. 3. In each case, the He II zone is centered closely on a specific star; in one, an O star, in two, Wolf-Rayet stars, and in N159F the He II is associated with the X-ray binary LMC X-1. N44C has diffuse X-ray emission and high-velocity gas, which suggests the presence of shocked gas. The close correlation of the emission with specific stars argues that photoionization by the stars themselves is the excitation mechanism, and there is some evidence that this may be true for those nebulae ionized by WO stars and perhaps some rare high-excitation WN stars. If this is the case, the derived ratios of He+-ionizing photons to H-ionizing photons yield effective temperatures greater than 60,000 K. However, possible association with diffuse X-ray emission and shocked gas confuses the picture, since some shock models also predict strong He II emission. A third possibility, suggested by Pakull & Angebault for N159F, is that the He II emission results from photoionization by X-rays from massive X-ray binaries. The existence of these nebulae with He II emission increases the likelihood that the lambda-4686 emission frequently observed in dwarf emission-line galaxies is nebular in origin. This suggests that the radiation field associated with star-forming regions can be harder than previously suspected, and reopens the question of whether photoionization by stars in young galaxies can account for the ionization observed in QSO absorption-line systems. It also raises questions regarding the usefulness of emission-line diagnostics in giant H II regions for exploring the properties of the massive star IMF.