The role of mergers in the obscured phase of stellar mass assembly: a first glimpse from Euclid

Galaxy mergers play a key role in the formation of hierarchical structures, driving stellar mass growth, morphological transformations, and AGN activity. With the advent of JWST and Euclid, it is now possible to trace these processes to earlier cosmic times and across wider areas. In particular, dusty red galaxies, such as HIEROs, represent a heavily obscured but crucial phase of galaxy evolution. This thesis presents the first statistical study of mergers in dusty galaxies using the Euclid Deep Field North from the Cosmic Dawn Survey. From a parent sample of approximately 3 million sources, we select 5750 HIEROs and identify close companions within projected separations of 5–50, 5–80, and 5–100 kpc, up to redshift z ≈ 9. We find that the pair fraction peaks at z ≈ 1 and declines toward z ≈ 5, with no robust detections at higher redshifts. Surprisingly, the most massive HIEROs exhibit fewer companions than expected. A conservative estimate suggests that mergers contribute approximately 5–40% to stellar mass assembly in these systems. While mass incompleteness limits firm conclusions, the results highlight the importance of mergers in the early universe and the power of future Euclid datasets to reveal their role.