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

Context. The SPT0311-58 system resides in a massive dark-matter halo at z similar to 6.9. It hosts two dusty galaxies (E and W) with a combined star formation rate (SFR) of similar to 3500 M-circle dot 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 similar to 17 x 17 kpc2 around this object, with a spatial resolution of similar to 0.5 kpc. Results. These observations reveal ten new galaxies at z similar to 6.9 characterised by dynamical masses spanning from similar to 109 to 1010 M-circle dot and a range in radial velocity of similar to 1500 km s(-1), in addition to the already known E and W galaxies. The implied large number density (phi similar to 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 similar to(5 +/- 3) x 1012 M-circle dot, 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 H alpha-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]/H beta) varying from similar to 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 (similar to 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.