S21B0112

We propose 1 night of FOCAS-IFU observation to study the origin of a large-scale star-bursting structure at z=1.06, consisting of a blue ring encircling a massive red galaxy, and a companion brightest cluster galaxy (BCG), all located at the center of a distant, low-mass cluster (HSCJ0220−0448). This is a rare discovery enabled by the deep (i∼26 AB mag), wide (1300 deg²) Subaru Hyper Suprime-Cam (HSC) survey, and offers an exciting opportunity to study a short-lived but important phase of cluster core formation. Data from archival Spitzer, Herschel, and our own ALMA programs reveal an integrated obscured star formation rate of ∼ 200 M_⊙/yr and ample amount of molecular gas. The proposed FOCAS observations will offer the missing information of unobscured SFR, stellar age, stellar metallicity, and, most importantly, the stellar dynamics of all the constituent systems, allowing us to pin down the origin and the future of this structure (with the help of cosmological hydrodynamical numerical simulations), representing a unique window to reveal the on-going formation of a cluster core.

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