Identifying a protocluster formation at z ~ 4.5 in the COSMOS field: an extension of Taralay protocluster, traced by Ly-alpha emitters surrounding a submm galaxy

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 towards the recently identified Taralay protocluster. We use combined wide-band and narrow-band optical photometry to identify Ly$α$ emitters (LAEs) within a 21 cMpc radius from the submm 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 narrow-band excess objects and we use the COSMOS2020 photometric redshift catalog to eliminate potential low-redshift interlopers whose line emission (e.g. [OIII] 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 cMpc³, probably evolving into a moderate-mass cluster (3 - 10 × 10¹⁴ \, M_) at z~ 0. This work supports the idea that submm sources, although offset from the major overdensity peaks, serve as traces of moderately massive, potentially infalling structures.