Kavli Affiliate: Yingjie Peng
| Summary:
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 $M_star gtrsim 10^8 , rm M_odot$ at $zsimeq 5-11$, by combining complementary theoretical approaches within $Λ$CDM. These include the DECODE semi-empirical model, the TNG100 and EAGLE hydrodynamical simulations, and the textscL-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 $sim 30%$ longer than those inferred from cosmological simulations including baryons. Galaxy major merger ($M_rm star,sat / M_rm star,cen > 0.25$) rates are high at cosmic dawn and increase towards earlier epochs, reaching $sim 1/rm Gyr$ at $zsim 5$ and peaking at $sim 3/rm Gyr$ at $zsim 8$. Despite these high rates, mergers contribute only $lesssim 10%$ of the total cosmic stellar mass density evolution. The ex-situ stellar mass fraction ranges from a few percent to $sim 20%$ for all galaxies at $zgtrsim 5$, 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.
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