2024MNRAS.533.2488M

We present JWST/NIRSpec integral-field spectroscopy observations of the z $\sim$ 9.11 lensed galaxy MACS1149-JD1, as part of the GA-NIFS programme. The data were obtained with both the G395H grating (R $\sim$ 2700) and the prism (R $\sim$ 100). This target shows a main elongated UV-bright clump and a secondary component detected in continuum emission at a projected distance of 2 kpc. The R2700 data trace the ionized-gas morpho-kinematics in between the two components, showing an elongated emission mainly traced by [O III] $\lambda$5007. We spatially resolve [O II] $\lambda \lambda $3726,3729, [O III] $\lambda \lambda$4959,5007, and [O III] $\lambda$4363, which enable us to map the electron density (n$_{\rm e} \sim 1.0 \times 10^3$ cm$^{-3}$), temperature (T$_{\rm e} \sim 1.6 \times 10^4$ K), and direct-method gas-phase metallicity (-1.2 to -0.7 dex solar). A spatially resolved full-spectrum modelling of the prism indicates a north-south gas metallicity and stellar age gradient between the two components. We found 3$\sigma$ evidence of a spatially resolved anticorrelation of the gas-phase metallicity and the star formation rate density, which is likely driven by gas inflows, enhancing the star formation in JD1. We employ high-z sensitive diagnostic diagrams to rule out the presence of a strong AGN in the main component. These findings show the unambiguous presence of two distinct stellar populations, with the majority of the mass ascribed to an old star formation burst, as suggested by previous works. We disfavour the possibility of a rotating-disc nature for MACS1149-JD1; we favour a merger event that has led to a recent burst of star formation in two separate regions, as supported by high values of [O III] $\lambda$5007/$\mathrm{H}\,\beta$, ionized gas velocity dispersion, and gas-phase metallicity.