Not all protoclusters host evolved galaxies: Evidence for reduced environmental effects in a lower halo mass protocluster at z =7.66

The progenitors of present-day galaxy clusters offer crucial insight into how galaxies and large-scale structure co-evolve in the early Universe. We present -emitters and five additional photometric candidates within a 0. 3 arcmin2 (1. 5 /NIRCam grism spectroscopy of the photometrically identified z=7. 66 protocluster core in the SMACS J0723. 3-7327 lensing field, We find six O III ̊m cMpc ²) region, corresponding to an overdensity of δ ∼ 200. Despite the extreme overdensity, the resident galaxies exhibit star formation histories, UV slopes, and neutral hydrogen column densities that are consistent with those of field galaxies at similar redshifts. This is in stark contrast to the consistently high neutral hydrogen column densities, old stellar populations, and large dust masses of galaxies within a z=7. 88 protocluster in the Abell 2744 field. Comparison with the TNG-Cluster and TNG300 simulations indicates a halo mass of ̊m log_ 10 (M_ 200 ̊m c ̊m M_⊙) = 11. 4 (M_ and implies that, on average, will evolve into a present-day Fornax-like cluster (̊m log_ 10 200 ̊m c, z=0 ̊m M_⊙) = 14. 3 (M_ The uniformly young, highly star-forming nature of the galaxy population of suggests that environmental effects only become significant above halo masses of ̊m log_ 10 200 ̊m c ̊m M_⊙) ≳ 11. 5. Comparison with other z protoclusters reveals that vigorous star formation persists in lower-mass protoclusters, whereas accelerated evolution and suppression of star formation emerge in more massive haloes. therefore represents an early stage of protocluster assembly, in which residence within an overdense environment still enhances star formation and feedback processes have yet to exert a significant influence.