GA-NIFS: The core of an extremely massive protocluster at the epoch of reionisation probed with JWST/NIRSpec

¹ Centro de Astrobiología, (CAB, CSIC–INTA), Departamento de Astrofísica, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: arribas@cab.inta-csic.es
² Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁴ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁵ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁶ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁷ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁸ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁹ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
¹⁰ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹¹ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹² European Space Agency, c/o STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁴ INAF – Osservatorio Astrofisco di Arcetri, largo E. Fermi 5, 50127 Firenze, Italy
¹⁵ AURA for European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21210, USA

Received: 2 December 2023
Accepted: 24 April 2024

Abstract

Context. The SPT0311–58 system resides in a massive dark-matter halo at z ∼ 6.9. It hosts two dusty galaxies (E and W) with a combined star formation rate (SFR) of ∼3500 M_⊙ yr⁻¹, mostly obscured and identified by the rest-frame IR emission. The surrounding field exhibits an overdensity of submillimetre sources, making it a candidate protocluster.

Aims. Our main goal is to characterise the environment and the properties of the interstellar medium (ISM) within this unique system.

Methods. We used spatially resolved low-resolution (R = 100) and high-resolution (R = 2700) spectroscopy provided by the JWST/NIRSpec Integral Field Unit to probe a field of ∼17 × 17 kpc² around this object, with a spatial resolution of ∼0.5 kpc.

Results. These observations reveal ten new galaxies at z ∼ 6.9 characterised by dynamical masses spanning from ∼10⁹ to 10¹⁰ M_⊙ and a range in radial velocity of ∼1500 km s⁻¹, in addition to the already known E and W galaxies. The implied large number density (ϕ ∼ 10⁴ Mpc⁻³) and the wide spread in velocities confirm that SPT0311–58 is at the core of a protocluster immersed in a very massive dark-matter halo of ∼(5 ± 3) × 10¹² M_⊙, and therefore represents the most massive protocluster ever found at the epoch of reionisation (EoR). We also studied the dynamical stage of its core and find that it is likely not fully virialised. The galaxies in the system exhibit a wide range of properties and evolutionary stages. The contribution of the ongoing Hα-based unobscured SFR to the total star formation (SF) varies significantly across the galaxies in the system. Their ionisation conditions range from those typical of the field galaxies at similar redshift recently studied with JWST to those found in more evolved objects at lower redshift, with log([OIII]/Hβ) varying from ∼0.25 to 1. The metallicity spans more than 0.8 dex across the FoV, reaching nearly solar values in some cases. The detailed spatially resolved spectroscopy of the E galaxy reveals that it is actively assembling its stellar mass, showing inhomogeneities in the ISM properties at subkiloparsec scales, and a metallicity gradient (∼0.1 dex kpc⁻¹) that can be explained by accretion of low metallicity gas from the intergalactic medium. The kinematic maps also depict an unsettled disc characterised by deviations from regular rotation, elevated turbulence, and indications of a possible precollision minor merger.

Conclusions. These JWST/NIRSpec IFS observations confirm that SPT0311–58 is at the core of an extraordinary protocluster, and reveal details of its dynamical properties. They also unveil and provide insights into the diverse properties and evolutionary stages of the galaxies residing in this unique environment.