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

Abstract

We report the discovery of a large-scale structure containing multiple overdensities at $z=4.54\pm 0.03$ 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$\alpha$ 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 $z\sim 0.3$) 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 $27\times 20\times 36$ cMpc${}^3$, probably evolving into a moderate-mass cluster ($3-10\times 10^{14}{M}_{\odot }$) at $z\sim 0$. This work supports the idea that submm sources, although offset from the major overdensity peaks, serve as traces of moderately massive, potentially infalling structures.

Accepted for publication in ApJ. 25 Pages, 14 Figures