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The origin of nebular HeII emission, which is frequently observed inlow-metallicity (O/H) star-forming galaxies, remains largely an unsolvedquestion. Using the observed anticorrelation of the integrated X-ray luminosityper unit of star formation rate ($L_X/{\rm SFR}$) of an X-ray binary populationwith metallicity and other empirical data from the well-studied galaxy I Zw 18,we show that the observed HeII 4686 intensity and its trend with metallicity isnaturally reproduced if the bulk of He$^+$ ionizing photons are emitted by theX-ray sources. We also show that a combination of X-ray binary populationmodels with normal single and/or binary stellar models reproduces the observed$I(4686)/I(H\beta)$ intensities and its dependency on metallicity and age. Weconclude that both empirical data and theoretical models suggest that high-massX-ray binaries are the main source of nebular HeII emission in low-metallicitystar-forming galaxies.

X-ray binaries as the origin of nebular HeII emission in low-metallicity star-forming galaxies