GA-NIFS: ISM properties and metal enrichment in a merger-driven starburst during the epoch of reionization probed with JWST and ALMA

We present deep JWST/NIRSpec integral-field spectroscopy (IFS) and ALMA [C II]λ158μm observations of COS-3018, a star-forming galaxy at z ∼ 6.85, as part of the GA-NIFS programme. Both G395H (R ∼ 2700) and PRISM (R ∼ 100) NIRSpec observations revealed that COS-3018 is comprised of three separate components detected in [O III]λ5007, which we dub as Main, North, and East, with stellar masses of 109.4±0.1, 109.2±0.07, 107.7±0.15 Mʘ. We detect [O III]λλ5007,4959, [O II]λλ3727,3729, and multiple Balmer lines in all three components together with [O III]λ4363 in the Main and North components. This allows us to measure an interstellar medium temperature of Te = 1.27±0.07 × 104 and Te = 1.6±0.14 × 104 K with densities of ne = 1250±250 and ne = 700±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 Te-method. We do not find any significant evidence of metallicity gradients between the components. Furthermore, we also detect [N II]λ6585, 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]λ5007 kinematics show that this system is merging, which is probably driving the ongoing, luminous starburst.