Poster Presentation

Discovery of a protocluster of massive quiescent galaxies at z = 2.77

Author(s): Kei Ito (U. Tokyo)

Presenter: Kei Ito (The University of Tokyo)

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~3. Here, we report the discovery of an overdense structure of massive quiescent galaxies with the spectroscopic redshift of z=2.77 in the COSMOS field, QO-1000. We first photometrically identify this structure as a 4.2sigma overdensity with 14 quiescent galaxies in 7x4 pMpc2 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 and located in a narrow redshift range (dz=0.03). Moreover, three of them are in the 1x1 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 at least 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(Mhalo/Msun) more than 13.2 from their stellar mass. Similar structures found in the IllustrisTNG simulation are expected to evolve into Coma-like massive galaxy clusters 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.