SIMBAD references

We present a sample of 2865 emission-line galaxies with strong nebular He_IIλ4686 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_IIλ4686/ H_β versus [N_II]λ6584/H_α 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_IIλ4686 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_IIλ4686 emission. We confirm previous findings that the models can predict He_IIλ4686 emission only for instantaneous bursts of 20 per cent solar metallicity or higher, and only for ages of ∼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_IIλ4686 emission. We discuss possible reasons for this and possible mechanisms for the He_IIλ4686 emission in these galaxies.