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The nature of the sources powering nebular HeII emission in star-forminggalaxies remains debated, and various types of objects have been considered,including Wolf-Rayet stars, X-ray binaries, and Population III stars. ModernX-ray observations show the ubiquitous presence of hot gas filling star-forminggalaxies. We use a collisional ionization plasma code to compute the specificHeII ionizing flux produced by hot gas and show that if its temperature is nottoo high (less than 2.5 MK), then the observed levels of soft diffuse X-rayradiation could explain HeII ionization in galaxies. To gain a physicalunderstanding of this result, we propose a model that combines thehydrodynamics of cluster winds and hot superbubbles with observed populationsof young massive clusters in galaxies. We find that in low-metallicitygalaxies, the temperature of hot gas is lower and the production rate of HeIIionizing photons is higher compared to high-metallicity galaxies. The reason isthat the slower stellar winds of massive stars in lower-metallicity galaxiesinput less mechanical energy in the ambient medium. Furthermore, we show thatensembles of star clusters up to 10-20 Myr old in galaxies can produce enoughsoft X-rays to induce nebular HeII emission. We discuss observations of thetemplate low-metallicity galaxy I Zw 18 and suggest that the HeII nebula inthis galaxy is powered by a hot superbubble. Finally, appreciating the complexnature of stellar feedback, we suggest that soft X-rays from hot superbubblesare among the dominant sources of HeII ionizing flux in low-metallicitystar-forming galaxies.

Ionization of HeII in star-forming galaxies by X-rays from cluster winds and superbubbles