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 had been no known structures identified by multiple 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 pMpc^2 from the COSMOS2020 catalog. We then securely confirm the spectroscopic redshifts of 4 quiescent galaxies by detecting multiple Balmer absorption lines with Keck/MOSFIRE. All the spectroscopically confirmed members are massive (log(M/M☉) > 11.0) and located in a narrow redshift range (2.76 < z < 2.79). Moreover, three of them are in the 1×1 pMpc^2 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 in the general field. In addition, we confirm that they form a red sequence in the J − Ks color. This structure's halo mass is estimated as log(M_halo/M☉) > 13.2 from their stellar mass. Similar structures found in the IllustrisTNG simulation are expected to evolve into massive galaxy clusters with log(M_halo/M☉) ≥ 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 the star-forming protoclusters and the 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