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

Abstract

We present deep JWST/NIRSpec integral-field spectroscopy (IFS) and ALMA [CII]$\hat{I}»$158$\hat{I}¼$m observations of COS-3018, a star-forming galaxy at z$\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 [OIII]$\hat{I}»$5008, which we dub as Main, North and East, with stellar masses of 10${}^{9.4\pm 0.1}$, 10${}^{9.2\pm 0.07}$, 10${}^{7.7\pm 0.15}$ M${}_{\odot }$. We detect [OIII]$\hat{I}»$5008, [OIII]$\hat{I}»\hat{I}»$3727,29 and multiple Balmer lines in all three components together with [OIII]$\hat{I}»$4363 in the Main and North components. This allows us to measure an ISM temperature of T${}_e$= 1.27$\pm 0.07\times 10^4$ and T${}_e$= 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$-method. We do not find any significant evidence of metallicity gradients between the components. Furthermore, we also detect [NII]$\hat{I}»$6585, one of the highest redshift detections of this emission line. We find that in a small, metal-poor clump 0.2 arcsec west of the North component, N/O is elevated compared to other regions, indicating that nitrogen enrichment originates from smaller substructures, possibly proto-globular clusters. [OIII]$\hat{I}»$5008 kinematics show that this system is merging, which is probably driving the ongoing, luminous starburst.

Submitted to MNRAS