2025MNRAS.540.2176C

We present an analysis of the rest-frame optical ($\lambda \simeq 3100-5600 \,$ Å) spectrum of a $\mathrm{log}_{10}(M_*/\mathrm{M_\odot }) = 8.6$ star-forming galaxy at $z=8.271$ (EXCELS-63107) from JWST/NIRSpec medium-resolution observations taken as part of the Early eXtragalactic Continuum and Emission Line Science (EXCELS) survey. The galaxy (EXCELS-63107) is compact, with a size consistent with local star-forming cluster complexes ($r_e < 200 \, \rm {pc}$), and exhibits an extremely steep UV continuum slope measured from JWST/NIRCam photometry ($\beta =-3.3\pm 0.3$). The JWST/NIRSpec G395M spectrum of EXCELS-63107 is notable for its strong [O III]$\lambda 4363$ auroral-line emission relative to the [O III]$\lambda 5007$ forbidden line. Via a detailed emission-line and photoionization-modelling analysis, we find that the observed properties of EXCELS-63107 are consistent with the presence of an ionizing source with an effective temperature of $T_{\rm eff} \gtrsim 80 \, 000\, \rm {K}$ heating ionized gas with a density of $n_e < 10^4 \, \rm {cm}^{-3}$ to a volume-averaged electron temperature of $T_e \simeq 34 \, 000\, \rm {K}$. Crucially, we find that stellar population models assuming a standard initial mass function (IMF) are not capable of producing the required heating. We determine an oxygen abundance of ${12+\mathrm{log(O/H)}= 6.89^{+0.26}_{-0.21}}$ ($\simeq 1.6$ per cent of solar) 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 $M > 50 \, \rm {M}_{\odot }$ 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.