Strongly star forming galaxies in the local Universe with nebular He II λ4686 emission

We present a sample of 2865 emission-line galaxies with strong nebular He II lambda 4686 emissions in the Sloan Digital Sky Survey Data Release 7 and use this sample to investigate the origin of this line in star-forming galaxies. We show that star-forming galaxies and galaxies dominated by an active galactic nucleus form clearly separated branches in the He II lambda 4686/H beta versus [N-II]lambda 6584/H alpha diagnostic diagram and derive an empirical classification scheme which separates the two classes. We also present an analysis of the physical properties of 189 star-forming galaxies with strong He II lambda 4686 emissions. These star-forming galaxies provide constraints on the hard ionizing continuum of massive stars. To make a quantitative comparison with observation, we use photoionization models and examine how different stellar population models affect the predicted He II lambda 4686 emission. We confirm previous findings that the models can predict He II lambda 4686 emission only for instantaneous bursts of 20 per cent solar metallicity or higher, and only for ages of similar to 4-5 Myr, the period when the extreme-ultraviolet continuum is dominated by emission from Wolf-Rayet stars. We find, however, that 83 of the star-forming galaxies (40 per cent) in our sample do not have Wolf-Rayet features in their spectra despite showing strong nebular He II lambda 4686 emission. We discuss possible reasons for this and possible mechanisms for the He II lambda 4686 emission in these galaxies.