Mergers may not be the main channel for galaxy stellar mass growth at redshift z 5: Predictions from cosmological hierarchical models and simulations

Mergers are a crucial channel of galaxy growth, but their role relative to star formation remains uncertain at high redshift. Here we investigate merger rates and merger-driven stellar mass assembly for galaxies with stellar mass M 10 8, M at z 5-11, by combining complementary theoretical approaches within Λ CDM. These include the DECODE semi-empirical model, the TNG100 and EAGLE hydrodynamical simulations, and the L-Galaxies semi-analytical model, together with the Millennium and TNG100-Dark simulations used to examine dark matter subhalo statistics and merger timescales. This compares models with different treatments of baryonic physics and galaxy-halo connections. We find that dark matter-only simulations predict merger timescales about 30% longer than those inferred from cosmological simulations including baryons. Galaxy major merger (M,sat / M,cen > 0.25) rates are high at cosmic dawn and increase towards earlier epochs, reaching about 1/ Gyr at z about 5 and peaking at about 3/ Gyr at z about 8. Despite these high rates, mergers contribute only 10% of the total cosmic stellar mass density evolution. The ex-situ stellar mass fraction ranges from a few percent to about 20% for all galaxies at z 5, across different models and simulations. Their broad agreement implies that stellar mass assembly at cosmic dawn is dominated by in-situ star formation rather than mergers. These findings suggest that the high pair fractions identified by JWST may not directly trace efficient ex-situ stellar mass growth, highlighting the need for physically motivated merger timescales to interpret galaxy assembly in the early Universe.