Mergers may not be the main channel for galaxy stellar mass growth at redshift $z\gtrsim 5$: 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 $M_{\star }\gtrsim {10}^8{\mathrm{M}}_{\odot }$ at $z\simeq 5-11$, by combining complementary theoretical approaches within $\mathrm{\Lambda }$CDM. These include the DECODE semi-empirical model, the TNG100 and EAGLE hydrodynamical simulations, and the $\text{textsc}L-Galaxies$ 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 $\sim 30\mathrm{\%}$ longer than those inferred from cosmological simulations including baryons. Galaxy major merger ($M_{\star ,\mathrm{s}\mathrm{a}\mathrm{t}}\mathrm{/}{M}_{\star ,\mathrm{c}\mathrm{e}\mathrm{n}}>0.25$) rates are high at cosmic dawn and increase towards earlier epochs, reaching $\sim 1\mathrm{/}\mathrm{G}\mathrm{y}\mathrm{r}$ at $z\sim 5$ and peaking at $\sim 3\mathrm{/}\mathrm{G}\mathrm{y}\mathrm{r}$ at $z\sim 8$. Despite these high rates, mergers contribute only $\lesssim 10\mathrm{\%}$ of the total cosmic stellar mass density evolution. The ex-situ stellar mass fraction ranges from a few percent to $\sim 20\mathrm{\%}$ for all galaxies at $z\gtrsim 5$, 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.