Dissecting the very metal-poor galaxy SBS0335-052E with MUSE: massive stars vs. nebular HeII emission

Nebular HeII emission implies the presence of energetic photons (E≽54 eV). Despite the great deal of effort dedicated to understanding HeII ionization, its origin has remained mysterious, particularly in metal-deficient star-forming galaxies. Unfolding HeII-emitting, metal-poor starbursts at z∼0 can yield insight into the powerful ionization processes occurring in the primordial universe. Here we present a study on the origin of the extended nebular HeII emission in SBS 0335-052E, one of the most metal-poor (Z ∼ 3% Z_⊙ HeII-emitter starbursts known locally. Based on optical VLT/MUSE spectroscopic and Chandra X-ray observations, and current stellar models we found that the HeII-ionization budget of SBS 0335-052E can only be produced by peculiar, nearly metal-free ionizing stars (called here "PopIII-like" stars) with a top-heavy initial mass function. This result is in line with recent simulations for PopIII star formation down to z=0.