Fig. 9.
Line ratios of He II-emitting galaxies that show strong C IV emission in their spectrum. Model predictions from BPASS are shown, which are able to reproduce the observed line ratios using star-formation activity alone. Also shown for comparison are line ratios measured in local low-metallicity galaxies by Berg et al. (2016) and Senchyna et al. (2019). When compared to the local galaxies, our sources at higher redshifts favour lower metallicities.
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