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

Abstract

The SPT0311-58 system resides in a massive dark-matter halo at z sim 6.9. It hosts two dusty galaxies (E and W) with a combined star formation rate (SFR) of sim 3500 mostly obscured and identified by the rest-frame IR emission. The surrounding field exhibits an overdensity of submillimetre sources, making it a candidate protocluster. Our main goal is to characterise the environment and the properties of the interstellar medium (ISM) within this unique system. 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 sim 17 times 17 kpc$^2$ around this object, with a spatial resolution of sim 0.5 kpc. These observations reveal ten new galaxies at zsim 6.9 characterised by dynamical masses spanning from sim 109 to 1010 and a range in radial velocity of sim 1500 in addition to the already known E and W galaxies. The implied large number density (phi sim $) and the wide spread in velocities confirm that is at the core of a protocluster immersed in a very massive dark-matter halo of sim (5 pm 3) times 1012 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 Halpha -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 OIII varying from sim 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 (sim 0.1 dex/kpc) 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. These JWST/NIRSpec IFS observations confirm that 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.