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 AGN and star formation to a still unknown degree. We thus investigate the impact of galaxy mergers on star formation and AGN activity using 3, 330 galaxies at z = 4. 5 − 8. 5 from eight JWST fields (CEERS, JADES GOODS-S, NEP-TDF, NGDEEP, GLASS, El-Gordo, SMACS-0723, MACS-0416), covering an unmasked area of 189 arcmin2. We focus on star formation rate (SFR) enhancement, AGN fraction, and AGN excess in close pairs defined by Δz 0. 3 and projected separations rp 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 rp 20 kpc, with values 0. 25 ± 0. 10 dex and 0. 26 ± 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 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 rp 50 kpc and Δz 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.