2025ApJ...991...35R

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 Lyα 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 $\bar{\delta }=3$ spanning a region of 27 × 20 × 36 cMpc³, probably evolving into a moderate-mass cluster ((3-10) × 10¹⁴M_⊙) 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.