This letter reports the discovery of an overdense structure of massive quiescent galaxies at redshift z = 2.77 in the COSMOS field, named QO-1000. Photometric analysis reveals a 4.2σ overdensity with 14 quiescent galaxies. Spectroscopic confirmation using Keck/MOSFIRE reveals four massive, quiescent galaxies within a narrow redshift range (2.76 < z < 2.79), three of which are clustered within 1 × 1 pMpc². The structure's halo mass is estimated to be log(Mhalo/M⊙) > 13.2. IllustrisTNG simulations suggest QO-1000 will evolve into a massive galaxy cluster at z = 0, indicating it's a mature protocluster in a transition phase between star-forming protoclusters and quenched galaxy clusters.

Kei Ito, Masayuki Tanaka, Francesco Valentino, Sune Toft, Gabriel Brammer, Katriona M L Gould, Olivier Ilbert, Nobunari Kashikawa, Mariko Kubo, Yongming Liang, Henry J Mccracken, John R Weaver