S22A0071

We propose to conduct MOSFIRE spectroscopy for an overdense region of massive (logM_⋆/M_⊙>10.5) quiescent galaxies at z=2.8, which is newly found from the latest COSMOS2020 catalog. Though many passive galaxies exist in local galaxy clusters, most known protoclusters are found by only star-forming galaxies. Protoclusters consisting of quiescent galaxies had not been found at z>2. The high-redshift structures of quiescent galaxies are unique and expected to provide an insight into the (proto)cluster evolution and the connection between the environment and the quenching. Our goals of this proposal are 1). to spectroscopically confirm the structure from Balmer absorption lines of 12 quiescent galaxies therein and 2). to explore their star formation history through the observed spectrum and plenty of multi-band photometry data. The derived star formation history will conclude the primordial environmental effect on the stellar age and constrain the progenitor of this structure. If this structure is spectroscopically confirmed, this is the most distant protocluster of quiescent galaxies, which will be an ideal laboratory for conducting the follow-up observation, such as by ALMA.

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