Signatures of Very Massive Stars in the Epoch of Reionization

Abstract

We present ultra-deep ($\simeq 20-30$ hours), rest-frame UV spectroscopy with NIRSpec/JWST of two UV-bright galaxies at $z\sim 8.7$, CEERS-1019 and CEERS-1025 ($Z_{\mathrm{neb}}\simeq 0.1Z_{\odot }$), obtained as part of the JWST Cycle 4 SPURS large program. The spectra reveal exceptionally strong P-Cygni profiles in wind lines (NV $\lambda$1240 and CIV $\lambda$1550) and significant broad and strong HeII $\lambda$1640 emission ($\mathrm{EW}\simeq 2-4$ A). We compare the observations with synthetic stellar population models at $Z_{\star }\simeq 0.1Z_{\odot }$, both including and excluding very massive stars (VMS). Models including VMS provide a markedly improved fit to the data relative to non-VMS models ($\Delta$AIC and $\Delta$BIC $>70$), 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 ($\simeq 1.5-2.0$Myr) and high ionizing photon production efficiencies ($\log {\xi }_{\mathrm{ion}}[\mathrm{Hzer}{\mathrm{g}}^{-1}]\gtrsim 25.8$), exceeding those inferred from non-VMS models by $\sim 0.1-0.2$ dex. These results provide evidence for an overabundance of VMS at high-$z$ with an IMF extending well beyond $100M_{\odot }$, and highlight their potential role in shaping the rest-frame UV spectra, chemical enrichment, and ionizing output of galaxies in the early Universe.

Accepted for publication in A&A Letters, 4 pages and 2 Figures (+Appendix)