We propose to obtain spectroscopic redshifts of four massive star-forming galaxies (SFGs) at z∼ 4.5 to confirm an extremely dense galaxy group with Subaru/FOCAS in MOS mode. Recently, a massive quiescent galaxy (QG) at z = 4.53 is spectroscopically confirmed in the COSMOS field, surrounded by four SFGs within 23″ (150 physical kpc) selected from highly accurate photometric redshifts. Only two QGs are confirmed at zspec > 4 from previous research, and their environments remain unexplored. This is the densest region of massive galaxies at z ∼ 4.5 in the COSMOS field and the first discovery of a very high-redshift group candidate for which the existence of a QG is confirmed. Here, we propose to perform a spectroscopic follow-up to measure the redshifts of these SFGs. From the Lyα line and/or Lyman break feature, we can strongly constrain their redshifts with only 1 half-night of observations. If we confirm consistent redshifts to the QG, this QG is located at the center of the over-dense region and will provide us with an unique opportunity to look into the quenching mechanism of QGs at high redshift, especially the role of the environment.
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