2025A&A...697A..89C

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 Lyα emission in the G140M spectrum, shifted by ∼450 km/s redwards of the systemic redshift. Combined with a modelling of the Lyα 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.