2025MNRAS.539.2463S

We present deep JWST/NIRSpec integral-field spectroscopy (IFS) and ALMA [C II]$\lambda $158$\mu$m observations of COS-3018, a star-forming galaxy at z $\sim$ 6.85, as part of the GA-NIFS programme. Both G395H (R $\sim$ 2700) and PRISM (R $\sim$ 100) NIRSpec observations revealed that COS-3018 is comprised of three separate components detected in [O III]$\lambda $5007, which we dub as Main, North, and East, with stellar masses of 10$^{9.4 \pm 0.1}$, 10$^{9.2 \pm 0.07}$, 10$^{7.7 \pm 0.15}$ $\mathrm{M}_\odot$. We detect [O III]$\lambda $$\lambda $5007,4959, [O II]$\lambda $$\lambda $3727,3729, and multiple Balmer lines in all three components together with [O III]$\lambda $4363 in the Main and North components. This allows us to measure an interstellar medium temperature of $T_\text{e}$ = 1.27$\pm 0.07\times 10^4$ and $T_\text{e}$ = 1.6$\pm 0.14\times 10^4$ K with densities of $n_{e}$ = 1250$\pm$250 and $n_{e}$ = 700$\pm$200 cm$^{-3}$, respectively. These deep observations allow us to measure an average metallicity of 12 + log(O/H) = 7.9-8.2 for the three components with the T$_{e}$-method. We do not find any significant evidence of metallicity gradients between the components. Furthermore, we also detect [N II]$\lambda $6585, one of the highest redshift detections of this emission line. We find that in a small, metal-poor clump 0.2 arcsec west of the North component, N/O is elevated compared to other regions, indicating that nitrogen enrichment originates from smaller substructures, possibly proto-globular clusters. [O III]$\lambda $5007 kinematics show that this system is merging, which is probably driving the ongoing, luminous starburst.