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ABSTRACT We present an analysis of the rest-frame optical (3100-5600 \, Å) spectrum of a log₁₀ (M_*/M_) = 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 (rₑ 200 \, pc), and exhibits an extremely steep UV continuum slope measured from JWST/NIRCam photometry (=-3. 3 0. 3). The JWST/NIRSpec G395M spectrum of EXCELS-63107 is notable for its strong O iii 4363 auroral-line emission relative to the O iii 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 ₄₅₅ 80 \, 000\, K heating ionized gas with a density of nₑ 10⁴ \, cm^-3 to a volume-averaged electron temperature of Tₑ 34 \, 000\, 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+log (O/H) = 6. 89^{+0. 26-₀. ₂₁} (1. 6 per cent of solar) which is one of the lowest directly constrained oxygen abundances measured in any galaxy to date, and 10 lower than is typical for z 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 10-30 \, excess of M 50 \, M 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.
Cullen et al. (2025) studied this question.