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Abstract
We present an analysis of the rest-frame optical (๐ โ 3100 โ 5600 ร
) spectrum of a log10 (๐โ/Mโ ) = 8.6 star-forming galaxy at ๐ง = 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 (๐๐ < 200 pc) and has an extremely steep UV continuum slope measured from JWST/NIRCam photometry (๐ฝ = โ3.3 ยฑ 0.3). The JWST/NIRSpec G395M
spectrum of EXCELS-63107 is notable for its strong [O iii]๐4363 auroral-line emission relative to the [O iii]๐5007 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 ๐eff โณ 80 000 K heating ionized gas with a density of ๐๐ < 104 cmโ3 to a volume-averaged electron temperature of ๐๐ โ 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
โ0.21 (โ 1.6 percent of solar) which is one of the lowest directly constrained oxygen abundances measured in any galaxy to date, and โ 10ร lower than is typical for ๐ง โ 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 10 โ 30 ร excess of ๐ > 50 Mโ 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.
EXCELS-63107 is a compact object consistent with the size of local star-forming cluster complexes (๐๐ < 200 pc) and has an extremely steep UV continuum slope measured from JWST/NIRCam photometry (๐ฝ = โ3.3 ยฑ 0.3). The JWST/NIRSpec G395M
spectrum of EXCELS-63107 is notable for its strong [O iii]๐4363 auroral-line emission relative to the [O iii]๐5007 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 ๐eff โณ 80 000 K heating ionized gas with a density of ๐๐ < 104 cmโ3 to a volume-averaged electron temperature of ๐๐ โ 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
โ0.21 (โ 1.6 percent of solar) which is one of the lowest directly constrained oxygen abundances measured in any galaxy to date, and โ 10ร lower than is typical for ๐ง โ 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 10 โ 30 ร excess of ๐ > 50 Mโ 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 language | English |
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Pages (from-to) | 2176-2194 |
Number of pages | 19 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 540 |
Issue number | 3 |
Early online date | 22 May 2025 |
DOIs | |
Publication status | Published - 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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Dive into the research topics of 'The JWST EXCELS survey: an extremely metal-poor galaxy at z = 8.271 hosting an unusual population of massive stars'. Together they form a unique fingerprint.Projects
- 3 Active
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The Origins of Massive Galaxies
Carnall, A. (Principal Investigator)
1/06/24 โ 31/05/29
Project: Research
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Exploring the formation and growth of the first galaxies
Dunlop, J. (Principal Investigator)
31/03/23 โ 30/03/28
Project: Research
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ICE: Inception of the Chemical Elements
Cullen, F. (Principal Investigator)
1/09/22 โ 31/08/27
Project: Research