The Pristine He II Emitter near GN-z11: Constraining the Mass Distribution of the First Stars

The properties of the first metal-free stars remain largely unknown, and so far, the only data-driven constraints on their initial mass function (IMF) come from near-field cosmology. Here, we interpret new observations of the C1 and C2 components of Hebe, the He II emitter near the galaxy GN-z11. Using a locally calibrated model, we robustly confirm the pristine (Population III, Pop III) nature of both components, showing that the measured upper limits on metal lines can only be reproduced by galaxies with >50% of their stellar mass in Pop III stars. We find that C1 is consistent with a purely Pop III system and adopt a simple parametric approach to infer the implications for the Pop III IMF and stellar mass. The observed He II/Hγ ratio excludes steep IMFs, favoring top-heavy distributions, especially for young stellar ages (≤1 Myr). Combined with the He II luminosity, this implies a total Pop III stellar mass of 2 · 104 < M⋆/M⊙ < 6 · 105. While degeneracies between IMF, stellar mass, and age remain, adopting the lower stellar masses predicted by simulations (M⋆ < 105 M⊙) strengthens the preference for top-heavy IMFs. Combining these results with near-field constraints, which instead exclude the flattest IMFs, we define a data-driven range of viable Pop III IMFs, linking characteristic mass and slope. This work demonstrates that direct observations of high-z Pop III systems can place independent constraints on the IMF of the first stars, opening a new window on their formation and properties.