Galaxy Mergers in the Epoch of Reionization II: Major Merger-Triggered Star Formation and AGN Activities at $z =$ 4.5-8.5

Galaxy mergers are a key driver of galaxy formation and evolution, including the triggering of AGN and star formation to a still unknown degree. We thus investigate the impact of galaxy mergers on star formation and AGN activity using a sample of 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, and MACS-0416), collectively covering an unmasked area of 189 arcmin$^2$. We focuses on star formation rate (SFR) enhancement, AGN fraction, and AGN excess in major merger ($μ> 1/4$) close-pair samples, defined by $Δz < 0.3$ and projected separations $r_p < 100$ kpc, compared to non-merger samples. We find that SFR enhancement occurs only at $r_p < 20$ kpc, with values of $0.25 \pm 0.10$ dex and $0.26 \pm 0.11$ dex above the non-merger medians for $z = [4.5, 6.5]$ and $z = [6.5, 8.5]$. No other statistically significant enhancements in galaxy sSFR or stellar mass are observed at any projected separation or redshift bin. We also compare our observational results with predictions from the SC-SAM simulation and find no evidence of star formation enhancement in the simulations at any separation range. Finally, we examine the AGN fraction and AGN excess, finding that the fraction of AGNs in AGN-galaxy pairs, relative to the total AGN population, is $3.25^{+1.50}_{-1.06}$ times greater than the fraction of galaxy pairs relative to the overall galaxy population at the same redshift. We find that nearly all AGNs have a companion within 100 kpc and observe an excess AGN fraction in close-pair samples compared to non-merger samples. This excess is found to be $1.26 \pm 0.06$ and $1.34 \pm 0.06$ for AGNs identified via the inferred BPT diagram and photometric SED selection, respectively.

💡 Research Summary

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This paper presents the second installment of the “Galaxy Mergers in the Epoch of Reionization” series, focusing on the impact of major mergers on star formation and active galactic nucleus (AGN) activity at redshifts 4.5 ≤ z ≤ 8.5. Using uniformly reduced JWST NIRCam imaging and NIRSpec spectroscopy from eight deep fields (CEERS, JADES‑GOODS‑S, NEP‑TDF, NGDEEP, GLASS, El‑Gordo, SMACS‑0723, and MACS‑0416), the authors assembled a sample of 3 330 galaxies covering 189 arcmin². Major‑merger candidates were defined as close pairs with stellar‑mass ratio μ > 1/4, photometric redshift separation Δz < 0.3, and projected separation rₚ < 100 kpc, yielding 1 024 pairs. A control sample of non‑merging galaxies was matched in redshift and stellar mass.

Star‑formation rates (SFRs) were derived from SED fitting, and specific SFRs (sSFR) and stellar masses (M★) were also measured. The key result is that SFR enhancement is detected only for the most compact pairs (rₚ < 20 kpc), where the median SFR is elevated by 0.25 dex (≈80 %) relative to the control. No statistically significant differences are found in sSFR or M★ at any separation, and beyond 20 kpc there is no measurable SFR boost. This suggests that at these early epochs galaxies already possess high gas fractions and vigorous star formation, so additional tidal torques from a merger only modestly increase the SFR when the nuclei are extremely close.

The authors compare these observations with predictions from the semi‑analytic SC‑SAM model. The simulation shows no SFR enhancement at any separation, highlighting a discrepancy between current theoretical treatments of high‑z merger‑driven star formation and the JWST data.

AGN activity is investigated using two independent selection methods: (1) an “inferred” BPT diagram based on rest‑frame optical emission‑line ratios derived from NIRSpec spectra, and (2) photometric SED fitting that identifies IR‑excess sources. Both methods identify ~3 % of the galaxy sample as AGN (112 objects). The fraction of AGN that reside in galaxy pairs is 3.25 × (±1.06/ +1.50) higher than the fraction of all galaxies found in pairs, indicating a strong association between merging and AGN triggering. Remarkably, 94 % of AGN have a companion within 100 kpc. An “AGN excess” is quantified: the AGN fraction in close‑pair subsamples is 1.26 ± 0.06 (BPT) and 1.34 ± 0.06 (SED) times larger than in the non‑merger sample, confirming a distance‑dependent rise in AGN incidence that mirrors results at lower redshift.

Statistically, the study benefits from a large, multi‑field sample, reducing random errors and cosmic variance. However, the reliance on photometric Δz < 0.3 introduces line‑of‑sight contamination; future work with spectroscopic redshifts will tighten pair identification. The AGN diagnostics, while robust, could be affected by dust obscuration or metallicity variations, especially in dense environments.

In summary, the paper demonstrates that major mergers at z ≈ 5–8 only modestly boost star formation when the galaxies are within ~20 kpc, but they strongly enhance the likelihood of hosting an AGN, with a clear excess that grows toward smaller separations. Current semi‑analytic models fail to reproduce the observed SFR boost, indicating that the physics of gas inflow, feedback, and black‑hole fueling in the early universe are not yet fully captured. The authors advocate for deeper NIRSpec spectroscopy and complementary ALMA observations to map molecular gas reservoirs, which will be essential for refining theoretical models of merger‑driven galaxy evolution during the epoch of reionization.