Photometrically selected protocluster candidates at $z\sim 9-10$ in the JWST COSMOS-Web field

Abstract High-redshift protoclusters are crucial for understanding the formation of galaxy clusters and the evolution of galaxies in dense environments. The James Webb Space Telescope (JWST), with its unprecedented near-infrared sensitivity, enables the first exploration of protoclusters beyond $ z \gt 10 $ . Among JWST surveys, COSMOS-Web Data Release 0.5 offers the largest area ( $\sim 0.27$ deg $^2$ ), making it an optimal field for protocluster searches. In this study, we searched for protoclusters at $ z \sim 9-10 $ using 366 F115W dropout galaxies. We evaluated the reliability of our photometric redshift by validation tests with the JADES DR3 spectroscopic sample, obtaining the likelihood of falsely identifying interlopers as $\sim25\%$ . Overdensities ( $\delta$ ) are computed by weighting galaxy positions with their photometric redshift probability density functions, using a 2.5 cMpc aperture and a redshift slice of $\pm 0.5$ . We selected the most promising core galaxies of protocluster candidate galaxies with an overdensity greater than the 95th percentile of the distribution of 366 F115W dropout galaxies. The member galaxies are then linked within an angular separation of 7.5 cMpc to the core galaxies, finding seven protocluster candidates. These seven protocluster candidates have inferred halo masses of $ M_{\text{halo}} \sim 10^{11}\,{\rm M}_{\odot} $ . The detection of such overdensities at these redshifts provides a critical test for current cosmological simulations. However, confirming these candidates and distinguishing them from low-redshift dusty star-forming galaxies or Balmer-break galaxies will require follow-up near-infrared spectroscopic observations.