Signatures of Very Massive Stars in the Epoch of Reionization

TL;DR

Deep JWST UV spectra of two high-redshift galaxies reveal signatures of very massive stars, suggesting their significant role in early galaxy evolution and reionization.

Contribution

First direct evidence supporting the presence of very massive stars in galaxies during the epoch of reionization.

Findings

Models with VMS fit the data significantly better than non-VMS models.

VMS presence implies extremely young stellar ages (~1.5-2 Myr).

High ionizing photon production efficiencies suggest VMS influence 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 ($Z_{\rm neb} \simeq 0.1 Z_{\odot}$). 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$1640 emission ($\rm EW\simeq 2-4$ A). We compare the observations with synthetic stellar population models at $Z_{\star} \simeq 0.1 Z_{\odot}$, 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 ($\log \xi_{\rm ion} [\rm Hz erg^{-1}] \gtrsim 25.8$), 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 $100 M_{\odot}$, and highlight their potential role in shaping the rest-frame UV spectra, chemical enrichment, and ionizing output of galaxies in the early Universe.