2025MNRAS.536.2000L

The process of galaxy cluster formation likely leaves an imprint on the properties of its individual member galaxies. Understanding this process is essential for uncovering the evolutionary connections between galaxies and cosmic structures. Here, we study a sample of 10 protoclusters at $z\sim 2$-3 in different dynamical states that we estimate based on spectroscopic data of their members. We combine the dynamical information with Hubble Space Telescope imaging to measure galaxy sizes and pair fractions. Our analysis reveals a clear anticorrelation between the velocity dispersion of the protocluster and its galaxy pair fractions (indicative of merger rates). The velocity dispersion also anticorrelates with the dispersion in size among the member galaxies. These correlations may be explained by protoclusters in colder dynamical states maintaining a velocity dispersion and galaxy number density that boosts galaxy mergers, which in turn contributes to the structural expansion and compaction of galaxies. Our findings offer constraints for cosmological models regarding the evolution of galaxy morphology across different stages in the assembly of protoclusters.