JADES: The star formation and chemical enrichment history of a luminous galaxy at z ∼ 9.43 probed by ultra-deep JWST/NIRSpec spectroscopy

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
² Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
³ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK
⁴ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁵ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200, Copenhagen, Denmark
⁶ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
⁷ Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
⁸ INAF – Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy
⁹ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁰ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹¹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
¹² Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK
¹³ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
¹⁴ Centro de Astrobiología (CAB), CSIC–NTA, Cra. de Ajalvir Km. 4, 28850 - Torrejón de Ardoz, Madrid, Spain
¹⁵ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁶ School of Physics, University of Melbourne, Parkville, 3010, VIC, Australia
¹⁷ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) Australia
¹⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge MA 02138 USA
¹⁹ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
²⁰ AURA for European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive. Baltimore, MD, 21210, USA
²¹ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI, 53706 USA
²² Department of Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High Street, Santa Cruz CA 96054, USA
²³ NSF's National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ, 85719, USA
²⁴ NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada

^⋆ Corresponding author: mirko.curti@eso.org

Received: 7 July 2024
Accepted: 13 February 2025

Abstract

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.