We report on the spectroscopic confirmation of overdense regions of massive quiescent galaxies (QGs) in the early Universe with JWST/NIRSpec. Based on data from the DeepDive NIRSpec program and archival data from the Dawn JWST Archive, we confirm three QGs in the vicinity of Jekyll & Hyde, a pair of massive QG and a dusty star-forming galaxy, at z=3.71 and two QGs around SXDS-27434 at z=4.01. According to the analysis of galaxy number density with photometric redshifts, Jekyll & Hyde (SXDS-27434) are in an overdense region, where the number density of galaxies is three times higher than the average in the COSMOS (SXDS) field. SED fitting suggests that most of the QGs follow similar star formation histories and have consistent formation and quenching epochs. The same trend is observed in other proto-clusters hosting QGs that were already identified by ground-based telescopes, indicating that the large-scale environment plays an important role in the formation of QGs. In addition, JWST spectra reveal a broad Hα emission line from SXDS-27434 and faint emission lines from other three QGs, which are identified as AGN-driven based on their emission line ratios. The overdensity is also reproduced by the Illustris TNG300 simulation at z=3.71, in which the member QGs also have similar quenching epochs. These results suggest that large-scale structure may enhance merger activity and/or gas accretion and trigger AGN feedback, which simultaneously drives galaxy quenching in the overdensity.
@article{arxiv.2604.21007,
title = {DeepDive: Simultaneous Formation of Massive Quiescent Galaxies in High-Redshift Galaxy Proto-clusters},
author = {Takumi Kakimoto and Masayuki Tanaka and Kei Ito and Francesco Valentino and Makoto Ando and Gabriel Brammer and Massissilia L. Hamadouche and Vasily Kokorev and Jacqueline Antwi-Danso and William M. Baker and Daniel Ceverino and Andreas L. Faisst and Marion Farcy and Michaela Hirschmann and Christian Kragh Jespersen and Mariko Kubo and Allison W. S. Man and Masato Onodera and Rhythm Shimakawa and John R. Weaver and Po-Feng Wu and Pengpei Zhu},
journal= {arXiv preprint arXiv:2604.21007},
year = {2026}
}