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Arxiv: The JWST EXCELS survey: an extremely metal-poor galaxy at z=8.271 hosting an unusual population of massive stars Published: 1/19/2025 6:19:34 PM Updated: 1/19/2025 6:19:34 PM

Paper abstract: We present an analysis of the rest-frame optical (\lambda ~eq 3100-5600\,\r{A}) spectrum of a \mathrm{log}_{10}(M_*/\mathrm{M_\odot}) = 8.6star-forming galaxy at z=8.271 (EXCELS-63107) from JWST/NIRSpecmedium-resolution observations taken as part of the EXCELS survey. EXCELS-63107is a compact object consistent with the size of local star-forming clustercomplexes (r_e < 200 \, \rm{pc}) and has an extremely steep UV continuummeasured from JWST/NIRCam photometry (\beta=-3.3\pm0.3). The JWST/NIRSpecG395M spectrum of EXCELS-63107 is notable for its strong [OIII]\lambda4363auroral-line emission relative to the [OIII]\lambda5007 forbidden line. Via adetailed emission-line and photoionization-modelling analysis, we find that thethe observed properties of EXCELS-63107 are consistent with the presence of anionizing source with an effective temperature of T_{\rm eff} ~eq 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 ~eq 34 \, 000\,\rm{K}.Crucially, we find that stellar population models assuming a standard IMF arenot capable of producing the required heating. We determine an oxygen abundanceof {12+\mathrm{log(O/H)}= 6.89^{+0.26}_{-0.21}} which is one of the lowestdirectly constrained oxygen abundances measured in any galaxy to date, and~eq 10 \times lower than is typical for z~eq8 galaxies with the samestellar mass. The extremely low metallicity of EXCELS-63107 places it in aregime in which theoretical models expect a transition to a top-heavy IMF, andwe speculate that a ~eq 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 mildlyenriched halo, are also consistent with the observations.