A Multiwavelength Assessment Disfavoring the X-Ray Binary Origin of He iii Regions in Metal-poor Star-forming Dwarf Galaxies

Recent observations of metal-poor, star-forming dwarf galaxies reveal He III regions, traced by nebular He ii λ 4686 emission, that require a strong source of extreme-ultraviolet (EUV) radiation. The origin of this hard ionizing radiation remains poorly understood, as standard stellar populations fail to account for it, posing key implications for the understanding of early galaxy formation. We present a systematic Chandra X-ray study of 21 nearby ( z < 0.1) star-forming galaxies with He ii emission but lacking Wolf–Rayet spectral signatures. Using 7 new and 36 archival Chandra X-ray observations, combined with optical stellar population synthesis modeling, we constrain the ionizing continuum required to sustain the observed He ii line, the ionizing continuum available from X-ray objects, and the properties of the host H II regions. We find that the inferred EUV output from accreting X-ray sources in our sample is systematically lower than what is required to produce the observed He ii emission. This discrepancy cannot be attributed to an anomalously low number or luminosity of X-ray sources. These results indicate that accreting X-ray sources alone cannot account for the observed He II –ionizing photon budget. Instead, these results are consistent with an evolutionary picture in which binary systems contribute to He II ionization through multiple stages and channels—from binary stripped helium stars, to supernova remnants and shocks, and ultimately to compact objects and accretion-powered X-ray binaries.