A first measurement of galaxy merger rate increasing in dynamically colder protoclusters at cosmic noon

TL;DR

This study measures galaxy merger rates in protoclusters at cosmic noon, finding that colder, less dynamically evolved protoclusters have higher merger rates, influencing galaxy evolution.

Contribution

It provides the first observational evidence linking protocluster dynamical state to galaxy merger rates and structural properties at high redshift.

Findings

Colder protoclusters have higher galaxy pair fractions.

Velocity dispersion inversely correlates with galaxy sizes.

Merger activity impacts galaxy structural evolution.

Abstract

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 ten protoclusters at z~2-3 in different dynamical states that we estimate based on spectroscopic data of their members. We combine the dynamical information with HST imaging to measure galaxy sizes and pair fractions. Our analysis reveals a clear anti-correlation between the velocity dispersion of the protocluster and its galaxy pair fractions (indicative of merger rates). The velocity dispersion also anti-correlates with the dispersion in size among of 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.