Signatures of very massive stars in the epoch of reionization

We present ultra-deep (≃20 − 30 hours) rest-frame UV spectroscopy with NIRSpec of two UV-bright galaxies at z ∼ 8.7, CEERS-1019 and CEERS-1025 ( Z neb ≃ 0.1 Z ⊙ ), obtained as part of the JWST Cycle 4 SPURS large program. The spectra reveal exceptionally strong P Cygni profiles in the wind lines (N V λ 1240 and C IV λ 1550) and significant broad and strong He II λ 1640 emission (EW ≃ 2 − 4 Å). We compared the observations with synthetic stellar population models at Z ★ ≃ 0.1 Z ⊙ , with and without very massive stars (VMSs). Models with VMSs provide a markedly improved fit to the data relative to non-VMS models (ΔAIC and ΔBIC > 70), which fail to reproduce the observed strengths of the wind features. A comparison with empirical spectra of VMS-dominated systems in the local Universe further supports this interpretation. The best-fit VMS models imply extremely young ages of the stellar populations (≃1.5–2.0 Myr) and highly ionizing photon production efficiencies (log ξ ion [Hz erg −1 ] ≳ 25.8) that exceed those inferred from non-VMS models by ∼0.1–0.2 dex. These results provide evidence of an overabundance of VMSs at high- z with an initial mass function extending well beyond 100 M ⊙ , and they highlight the potential role of VMSs in shaping the rest-frame UV spectra, chemical enrichment, and ionizing output of galaxies in the early Universe.