Study of the nature of He II nebular emission in the spectra of low-metallicity H II regions from the Sloan Digital Sky Survey

We studied a sample of galaxies from the Sloan Digital Sky Survey Data Release 7 that exhibited the He II λ 468.6 nm nebular line (an indicator of hard radiation in H II regions) in their spectra. The intensity of this line in the spectra of H II regions from our sample increased with decreasing metallicity, thus confirming the results of earlier studies. However, the theoretical models of population synthesis predict that the He II line intensity must decrease with decreasing metallicity. A possible connection between hard UV radiation and Wolf-Rayet stars was investigated. Only 30% of spectra from our sample exhibited both the nebular emission and the broad He II emission of Wolf-Rayet stars. This fact does not rule out the possibility that Wolf-Rayet stars serve as sources of hard ionizing radiation in some H II regions. However, other possible sources, such as the fast radiative shock waves, seem to be more likely to produce this hard ionizing radiation.