COSMOS2020: Discovery of a Protocluster of Massive Quiescent Galaxies at z = 2.77

Protoclusters of galaxies have been found in the last quarter-century. However, most of them have been found through the overdensity of star-forming galaxies, and there have been no known structures identified by more than two spectroscopically confirmed quiescent galaxies at z > 2.5. In this letter, we report the discovery of an overdense structure of massive quiescent galaxies with the spectroscopic redshift z = 2.77 in the COSMOS field, QO-1000. We first photometrically identify this structure as a 4.2σ overdensity with 14 quiescent galaxies in 7 × 4 pMpc² from the COSMOS2020 catalog. We then securely confirm the spectroscopic redshifts of four quiescent galaxies by detecting multiple Balmer absorption lines with Keck/MOSFIRE. All the spectroscopically confirmed members are massive ( $\mathrm{log}({M}_{\star }/{M}_{\odot })\gt 11.0$ ) and located in a narrow redshift range (2.76 < z < 2.79). Moreover, three of them are in the 1 × 1 pMpc² in the transverse direction at the same redshift (z = 2.760-2.763). Such a concentration of four spectroscopically confirmed quiescent galaxies implies that QO-1000 is >68 times denser than the general field. In addition, we confirm that they form a red sequence in the J - K _s color. This structure's halo mass is estimated as $\mathrm{log}({M}_{\mathrm{halo}}/{M}_{\odot })\gt 13.2$ from its stellar mass. Similar structures found in the IllustrisTNG simulation are expected to evolve into massive galaxy clusters with $\mathrm{log}({M}_{\mathrm{halo}}/{M}_{\odot })\geqslant 14.8$ at z = 0. These results suggest that QO-1000 is a more mature protocluster than the other known protoclusters. It is likely in a transition phase between star-forming protoclusters and quenched galaxy clusters.