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

F. Cullen*, A. C. Carnall, D. Scholte, D. J. McLeod, R. J. McLure, K. Z. Arellano-Cรณrdova, T. M Stanton, C. T. Donnan, J. S. Dunlop, A. E. Shapley, L. Barrufet, R. Begley, C. Bondestam, M. Cirasuolo, H. -H. Leung, C. L. Pollock, S. Stevenson

*Corresponding author for this work

Research output: Contribution to journal โ€บ Article โ€บ peer-review

Abstract

We present an analysis of the rest-frame optical (๐œ† โ‰ƒ 3100 โˆ’ 5600 ร…) spectrum of a log10 (๐‘€โˆ—/MโŠ™ ) = 8.6 star-forming galaxy at ๐‘ง = 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 (๐‘Ÿ๐‘’ < 200 pc) and has an extremely steep UV continuum slope measured from JWST/NIRCam photometry (๐›ฝ = โˆ’3.3 ยฑ 0.3). The JWST/NIRSpec G395M
spectrum of EXCELS-63107 is notable for its strong [O iii]๐œ†4363 auroral-line emission relative to the [O iii]๐œ†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 ๐‘‡eff โ‰ณ 80 000 K heating ionized gas with a density of ๐‘›๐‘’ < 104 cmโˆ’3 to a volume-averaged electron temperature of ๐‘‡๐‘’ โ‰ƒ 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
โˆ’0.21 (โ‰ƒ 1.6 percent of solar) which is one of the lowest directly constrained oxygen abundances measured in any galaxy to date, and โ‰ƒ 10ร— lower than is typical for ๐‘ง โ‰ƒ 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 โ‰ƒ 10 โˆ’ 30 ร— excess of ๐‘€ > 50 MโŠ™ 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.
Original languageEnglish
Pages (from-to)2176-2194
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Volume540
Issue number3
Early online date22 May 2025
DOIs
Publication statusPublished - 1 Jul 2025

Keywords / Materials (for Non-textual outputs)

  • galaxies: evolution
  • galaxies: formation
  • galaxies: high-redshift
  • galaxies: abundances
  • galaxies: starburst
  • dark ages
  • reionization
  • first stars

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