Kehrig, C.C.KehrigVílchez, J. M.J. M.VílchezGuerrero, M. A.M. A.GuerreroIglesias-Páramo, J.J.Iglesias-PáramoHUNT, Leslie KippLeslie KippHUNTDuarte-Puertas, S.S.Duarte-PuertasRamos-Larios, G.G.Ramos-Larios2020-11-132020-11-1320180035-8711http://hdl.handle.net/20.500.12386/28331SBS 0335 - 052E, one of the most metal-poor (Z ∼ 3-4{{ per cent}} Z<SUB>☉</SUB>) 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<SUP>+</SUP> 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<SUP>-5</SUP> 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.STAMPAenArticle10.1093/mnras/sty19202-s2.0-85056797303000442567900073https://academic.oup.com/mnras/article-abstract/480/1/1081/5056186?redirectedFrom=fulltext2018MNRAS.480.1081KFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation