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We analysed ultra-deep JWST observations of the galaxy JADES-GS-z9-0 at z = 9.4327, and derived detailed stellar and interstellar medium (ISM) properties of this luminous ( M UV =−20.43) high-redshift system. Complementary information from NIRCam imaging and NIRSpec spectroscopy (both low and medium resolution) reveal a compact system ( R e ∼110 pc) characterised by a steeply rising star formation history, which is reflected in the inferred young stellar age ( t ∼3 Myr, light-weighted), high star formation rate surface density (Σ SFR ∼72 M ⊙ yr −1 kpc −2 ), high ionisation parameter (log(U) ∼−1.5), low metallicity (12 + log(O/H) ∼7.5), and low carbon-to-oxygen abundance (C/O =−0.64). Leveraging the detection of N III ] λ1750 we derived a nitrogen-to-oxygen abundance (N/O ∼0) higher than the plateau followed by low-redshift galaxies of similar metallicity, possibly revealing the imprint from (very) massive stars on the ISM enrichment and favouring a top-heavy initial mass function (IMF) scenario. Massive stars powering a hard radiation field are also required to explain the rest-frame UV line ratios, though the presence of the high-excitation Ne V λ3426 emission line possibly hints at additional ionisation from an active galactic nucleus (AGN). We also report the tentative detection of Ly α emission in the G140M spectrum, shifted by ∼450 km/s redwards of the systemic redshift. Combined with a modelling of the Ly α spectral break, we rule out the presence of very high column densities of neutral gas pertaining to local absorbers, as well as any extended surrounding ionised bubbles, suggesting that JADES-GS-z9-0 has not yet significantly contributed to cosmic reionisation.
Curti et al. (Mon,) studied this question.
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