COSMOS2020: Discovery of a protocluster of massive quiescent galaxies at $z=2.77$

Understand

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 $4.2\sigma$ overdensity with 14 quiescent galaxies in $7\times4\ {\rm pMpc^2}$ from the COSMOS2020 catalog.