2025ApJ...985...83M

We present new JWST NIRSpec integral field unit (IFU) G395H/F290LP observations of a merging galaxy system at z = 7.88, part of A2744-z7p9OD, the most distant protocluster to date. The IFU cube reveals [O III]_λ5007 emissions in two previously known galaxies, ZD3 and ZD6, and a newly identified galaxy, ZD12, at z_spec = 7.8762. One of the detected [O III]-emitting regions has a detection of the auroral [O III]_λ4363 line, allowing us to derive a direct metallicity of ${\rm{log}}({\rm{O}}/{\rm{H}})$ + 12 = 7.4 ± 0.2, while metallicities in other regions are measured using strong-line calibration methods. We find large deviations within the measured metallicity (${\rm{\Delta }}\mathrm{log}({\rm{O}}/{\rm{H}})\sim 1$), which suggests a fast chemical enrichment from intense star formation and merger-driven growth, as expected in early galaxies. Our analysis shows that metal-poor regions could easily be outshone by more enriched regions, posing a challenge for spectroscopic analysis based on integrated light (i.e., NIRSpec/MSA) against identifying metal-free star formation in the early Universe. NIRCam imaging reveals seven UV-bright clumps in ZD12, in a stellar mass range of $\mathrm{log}{M}_{* }/{M}_{\odot }\sim 7.6$-8.9. Four of them are unresolved (≲100 pc) and intensely star-forming (>30 M_⊙ yr^–1 kpc^–2), likely contributing to the scatter in metallicity by producing an ideal environment for rapid chemical cycles. Lastly, we revisit the nature of the host protocluster by including new member galaxies identified here and in the literature, and obtain a local overdensity factor $\delta =4{4}_{-31}^{+89}$, a total halo mass ${M}_{{\rm{h}}}=5.{8}_{-0.3}^{+0.2}\times 1{0}^{11}\,{M}_{\odot }$, and a formal velocity dispersion 1100 ± 500 km s^–1.