S21B0125

We propose SWIMS medium-band survey of well-evolved massive galaxies toward a recently discovered proto-cluster of z=4.6 submillimeter galaxies (SMGs) in the COSMOS field, which is likely to be a progenitor of the present-day’s ∼10¹⁵M_⊙ cluster. This structure is a significant concentration of massive starburst galaxies and thus is a good laboratory to test expected early quenching of star-formation in over-dense regions at z>4. Based on the COSMOS photo-z galaxy catalog, we found that 8 quiescent galaxy candidates at z∼ 4.6 are located in the proto-cluster region. The SWIMS K₁ and K₂-band filters enable us to identify quiescent or post-starburst galaxies at z=4.6 by tracing Balmer or 4000Å break and to measure the quiescent fraction in the SMG proto-cluster. Even if we could not find any well-evolved massive galaxies, such null result would suggest that the 6 SMGs are the most massive galaxies in this structure and progenitors of the brightest cluster galaxies. This program will provide us with new insights into the passivization process of massive galaxies in over-dense regions in the early Universe.

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