Constraining the major merger history of $z \sim 3-9$ galaxies using JADES: dominant in-situ star formation

Abstract

We present a comprehensive analysis of galaxy close-pair fractions and major merger rates to evaluate the importance of mergers in the hierarchical growth of galaxies over cosmic time. This study focuses on the previously poorly understood redshift range of $z \approx 3-9$ using JADES observations. Our mass-complete sample includes primary galaxies with stellar masses of ${\rm log}(M_\star/{\rm M_\odot}) = [8, 10]$, having major companions (mass ratio $\geq 1/4$) selected by $5-30$ pkpc projected separation and redshift proximity criteria. Pair fractions are measured using a statistically robust method incorporating photometric redshift posteriors and available spectroscopic data. The pair fraction evolves with redshift and shows dependence on the stellar mass: at ${\rm log}(M_\star/{\rm M_\odot}) = [8.0, 8.5]$ there is an increase up to $z\sim5-6$, followed by a turnover; while at higher stellar masses there is a flattening and weak decline with increasing redshift. Similarly, the derived galaxy major merger rate increases and flattens beyond $z \sim 6$ to $2-8~{\rm Gyr^{-1}}$ per galaxy, showing a weak scaling with stellar mass, driven by the evolution of the galaxy stellar mass function. A comparison between the cumulative mass accretion from major mergers and the mass assembled through star formation indicates that major mergers contribute approximately $3-13\%$ to the total mass growth over the studied redshift range, which is in agreement with the ex-situ mass fraction estimated from our simple numerical model. These results highlight that major mergers contribute little to the direct stellar mass growth compared to in-situ star formation but could still play an indirect role by driving star formation itself.