GA-NIFS : witnessing the complex assembly of a star-forming system at z = 5.7

We present observations of the z ∼ 5.7 Lyman-break galaxy HZ10 with the JWST/NIRSpec integral field unit in high and low spectral resolution (G395H, spectral resolving power R ∼ 2700 and PRISM, R ∼ 100, respectively), as part of the GA-NIFS programme. By spatially resolving the source (spatial resolution ∼ 0.15″ or ∼ 0.9 kpc), we find three spatially and spectrally distinct regions of line emission along with one region of strong continuum emission, all within a projected distance of < 10 kpc. The R2700 data features strong detections in Hβ, [O III] λλ4959,5007, [N II] λλ6548,6584, Hα, and [S II] λλ6716,6731. The R100 data additionally contain a strong detection of the Ly α break, rest-frame UV and optical continuum, and [O II] λλ3726,3729. None of the detected lines present strong evidence for active galactic nucleus excitation from line diagnostic diagrams, and no high-ionization lines are detected. Using the detected lines, we constrain the electron density (log10 (ne/cm−3) ∼ 3) and metallicity (∼ 0.5 − 0.7 solar) in each component. Spaxel-by-spaxel fits reveal a strong east-west velocity gradient and significant line asymmetries (possibly indicating tidal features or outflows). The western component features a very red UV slope (βUV ∼ −0.9) and significant H α emission, suggesting an evolved population and active star formation. A comparison to high-resolution (∼ 0.3 arcsec or ∼ 1.8 kpc) [C II] 158 μm imaging obtained with the Atacama Large Millimetre/submillimetre Array (ALMA) reveals areas of dust obscuration. Altogether, these data suggest that HZ10 represents an ongoing merger, with a complex distribution of stars, gas, and dust < 1 Gyr after the big bang