The JWST EXCELS survey: an extremely metal-poor galaxy at $z=8.271$ hosting an unusual population of massive stars

Abstract

We present an analysis of the rest-frame optical ($\hat{I}»\simeq 3100-5600$Ã ) spectrum of a ${\log }_{10}(M_{\ast }/{\mathrm{M}}_{\odot })=8.6$ star-forming galaxy at $z=8.271$ from JWST/NIRSpec medium-resolution observations taken as part of the EXCELS survey. The galaxy (EXCELS-63107) is compact, with a size consistent with the size of local star-forming cluster complexes ($r_e<200\mathrm{pc}$) and has an extremely steep UV continuum measured from JWST/NIRCam photometry (${\hat{I}}^2=-3.3\pm 0.3$). The JWST/NIRSpec G395M spectrum of EXCELS-63107 is notable for its strong [OIII]$\lambda 4363$ auroral-line emission relative to the [OIII]$\lambda 5007$ forbidden line. Via a detailed emission-line and photoionization-modelling analysis, we find that the the observed properties of EXCELS-63107 are consistent with the presence of an ionizing source with an effective temperature of $T_{\mathrm{eff}}\gtrsim 80000\mathrm{K}$ heating ionized gas with a density of $n_e<10^4{\mathrm{cm}}^{-3}$ to a volume-averaged electron temperature of $T_e\simeq 34000\mathrm{K}$. Crucially, we find that stellar population models assuming a standard IMF are not capable of producing the required heating. We determine an oxygen abundance of $12+\log (O/H)=6.89_{-0.21}^{+0.26}$ which is one of the lowest directly constrained oxygen abundances measured in any galaxy to date, and $\simeq 10\times$ lower than is typical for $z\simeq 8$ galaxies with the same stellar mass. The extremely low metallicity of EXCELS-63107 places it in a regime in which theoretical models expect a transition to a top-heavy IMF, and we speculate that a $\simeq 10-30\times$ excess of $M>50{\mathrm{M}}_{\odot }$ stars is one plausible explanation for its observed properties. However, more exotic scenarios, such as Pop III star formation within a mildly enriched halo, are also consistent with the observations.

18 pages, 11 figures, updated to the final MNRAS accepted version