Galaxy mergers in the epoch of reionization – II. Merger-triggered star formation and AGN activities at z = 4.5−8.5

ABSTRACT Galaxy mergers are key drivers of galaxy formation and evolution, including triggering active galactic nucleus (AGN) and star formation to a still unknown degree. We thus investigate the impact of galaxy mergers on star formation and AGN activity using 3330 galaxies at $z = 4.5 {\text{-}} 8.5$ from eight James Webb Space Telescope fields (CEERS, JADES GOODS-S, NEP-TDF, NGDEEP, GLASS, El-Gordo, SMACS-0723, and MACS-0416), covering an unmasked area of 189 arcmin$^2$. We focus on star formation rate (SFR) enhancement, AGN fraction, and AGN excess in close pairs defined by $\Delta z < 0.3$ and projected separations $r_p < 100$ kpc, relative to non-merger samples. Close pairs with mass ratios greater than 1:4 are used for the SFR–enhancement analysis, whereas no mass–ratio constraint is applied for the AGN fraction and AGN excess measurements. We find SFR enhancement occurs only at $r_p < 20$ kpc, with values $0.25 \pm 0.10$ dex and $0.26 \pm 0.11$ dex above non-merger medians for $z = [4.5, 6.5]$ and $z = [6.5, 8.5]$, respectively. No other statistically significant enhancements in galaxy sSFR or stellar mass are observed at any projected separation or redshift. We also compare observational results with predictions from the SC-SAM simulation, finding no evidence of star formation enhancement in simulations at any separation. Lastly, we examine the fraction and excess of AGNs identified through photometric spectral energy distributions (SED) fitting (Type-I) and BPT diagnostics (Type-II). We find $48^{+33}_{-7}\%$ of Type-I and $44^{+26}_{-20}\%$ of Type-II AGNs have a close companion within $r_p < 50$ kpc and $\Delta z < 0.3$. Furthermore, $73^{+27}_{-32}\%$ of AGNs have companions within 100 kpc. Relative to isolated galaxies, we measure an AGN excess factor of $1.26^{+0.15}_{-0.04}$ for Type-I and $1.34^{+0.23}_{-0.11}$ for Type-II AGNs in close pairs, suggesting notable AGN enhancement in galaxy pairs at these higher redshifts.