What is Powering the Enigmatic He II Emitter Hebe : The First Stars or Black Holes?

Abstract Recent high-resolution spectroscopy with the James Webb Space Telescope has confirmed the presence of a strong He ii λ 1640-emitting clump in the vicinity of GN-z11, with only upper limits on its metallicity. To explain the peculiar properties of this source, now termed Hebe , a cluster of metal-free, Population III (Pop III) stars has been invoked. A less likely source for the hard UV ionizing radiation could be an accreting supermassive black hole (SMBH) embedded inside Hebe . We here provide further constraints on what could power the observed emission lines in Hebe . Comparing with cosmological simulations of Pop III star cluster formation, we assess the maximum Pop III stellar mass that could plausibly form at the location of Hebe , finding stellar masses of a few 10 5 M ⊙ , consistent with those inferred from the observations. Modeling the continuum spectral energy distribution arising from an accreting SMBH, we derive He ii and H i ionizing rates and the resulting recombination line luminosities, providing a less natural fit for the combined observations. We thus confirm the interpretation of Hebe as a remarkable, primordial object, with the most plausible power source provided by a massive cluster of Pop III stars, at the limit of what is allowed within the standard model of first star formation.