Enhanced Merger Fractions in a Reionization-era Protocluster

Abstract Mergers play a critical role in galaxy evolution, but their relationship to their surrounding environments is unexplored at high redshift. We investigate the galaxy merger rate among 124 [O iii ] emitters at 5.3 < z < 6.9 as a function of local galaxy density. Identified in the ASPIRE JWST/NIRCam grism survey, we investigate three density regimes: a z = 6.6 quasar-centered protocluster, two overdensities at z = 5.4 and z = 6.2, and field galaxies. We evaluate merger candidates through close pair and morphological criteria in NIRCam imaging, finding that the z = 6.6 protocluster contains the highest fraction of galaxies meeting either criterion. We observe a >3 σ enhancement of the merger fraction amongst all three overdense structures compared to the field. Eleven galaxies are classified as “active mergers” satisfying both merger criteria, all of which occur within the overdensity samples. We conclude that environment affects the merger rates of galaxies at z > 6, leading to increased specific star formation at the 4 σ level.