Identifying a Protocluster Formation at z ∼ 4.5 in the COSMOS Field: An Extension of the Taralay Protocluster, Traced by Lyα Emitters Surrounding a Submillimeter Galaxy

Abstract We report the discovery of a large-scale structure containing multiple overdensities at z = 4.54 ± 0.03 in the COSMOS field, with the most prominent one likely infalling toward the recently identified Taralay protocluster. We use combined wideband and narrowband (NB) optical photometry to identify Ly α emitters (LAEs) within a 21 cMpc radius from the submillimeter source J1000+0234, at z = 4.54, to identify typical star-forming galaxies that may trace an underlying structure. Our approach selects line emitters as NB excess objects and we use the COSMOS2020 photometric redshift catalog to eliminate potential low-redshift interlopers whose line emission (e.g., O III at z ∼ 0.3) might be responsible for the observed excess in the narrow band. In comparison with the LAE density in the field, our results point to a mean LAE number overdensity of δ ¯ = 3 spanning a region of 27 × 20 × 36 cMpc 3 , probably evolving into a moderate-mass cluster ((3–10) × 10 14 M ⊙ ) at z ∼ 0. This work supports the idea that submillimeter sources, although offset from the major overdensity peaks, serve as traces of moderately massive, potentially infalling structures.