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

We present observations of the z~5.7 Lyman-break galaxy HZ10 with the JWST/NIRSpec IFU in high and low spectral resolution (G395H, R~2700 and PRISM, R~100, respectively), as part of the GA-NIFS program. By spatially resolving the source, we find evidence for 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, [OIII]λλ4959,5007, [NII]λλ6548,6584, Hα, and [SII]λλ6716,6731. The R100 data additionally contains a strong detection of the Lyα break, rest-UV continuum, and [OII]λλ3726,3729. None of the detected lines present strong evidence for AGN excitation from line diagnostic diagrams, and no high-ionisation lines are detected. Using the detected lines, we constrain the electron density ( log₁₀( nₑ / cm⁻³)~2.5-3.3) and metallicity (~0.5-0.7 solar) in each component. Spaxel-by-spaxel fits of each cube reveal a strong east-west velocity gradient and significant line asymmetries (indicating tidal features or outflows). The western component features a very red UV slope (βUV~-1) and significant Hα emission, suggesting an evolved population and active star formation. From a comparison to high resolution [CII]158μm imaging obtained with the Atacama Large Millimetre/submillimetre Array (ALMA), we find that the continuum emitter is associated with a gas-poor stellar population. 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.