The extended He II λ4686 emission in the extremely metal-poor galaxy SBS 0335 - 052E seen with MUSE

SBS 0335 - 052E, one of the most metal-poor (Z ∼ 3-4{{ per cent}} Z_⊙) He II-emitter starbursts known in the nearby Universe, is studied using optical VLT/MUSE spectroscopic and Chandra X-ray observations. We spatially resolved the spectral map of the nebular He II λ4686 emission from which we derived for the first time the total He II-ionizing energy budget of SBS 0335 - 052E. The nebular He II line is indicative of a quite hard ionizing spectrum with photon energies > 4 Ryd, and is observed to be more common at high-z than locally. Our study rules out a significant contribution from X-ray sources and shocks to the He II photoionization budget, indicating that the He⁺ excitation is mainly due to hot stellar continua. We discovered a new WR knot, but we also discard single Wolf-Rayet stars as the main responsible for the He II ionization. By comparing observations with current models, we found that the He II-ionization budget of SBS 0335 - 052E can only be produced by either single, rotating metal-free stars or a binary population with Z ∼ 10^-5 and a `top-heavy' initial mass function. This discrepancy between the metallicity of such stars and that of the H II regions in SBS 0335 - 052E is similar to results obtained by Kehrig et al. for the very metal-deficient He II-emitting galaxy IZw18. These results suggest that the He II ionization is still beyond the capabilities of state-of-the-art models. Extremely metal-poor, high-ionizing starbursts in the local Universe, like SBS 0335 - 052E, provide unique laboratories for exploring in detail the extreme conditions likely prevailing in the reionization era.