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

We present an analysis of the rest-frame optical (𝜆 ≃ 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 EXCELS survey.
EXCELS-63107 is a compact object consistent with the size of local star-forming cluster complexes (𝑟𝑒 < 200 pc) and has an extremely steep UV continuum slope measured from JWST/NIRCam photometry (𝛽 = −3.3 ± 0.3). The JWST/NIRSpec G395M
spectrum of EXCELS-63107 is notable for its strong [O iii]𝜆4363 auroral-line emission relative to the [O iii]𝜆5007 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 𝑇eff ≳ 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 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 percent 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.