Distribution of He II EW and FWHM for sources in our sample. Marked by the dashed line is the cut in FWHM = 1000 km s⁻¹ introduced to create subsamples for stacking analysis. This separation has been motivated by previously observed (and expected) He II FWHM from narrow and broad emitters – the broad He II can generally be explained by AGN and/or stellar winds, whereas narrow He II emission is generally attributed to low-metallicity stars (see Cassata et al. 2013). Sources with strong C IV emission or X-ray detection that have been tentatively classified as AGNs are excluded from further analysis, and are marked by red circles.