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

ABSTRACT We present observations of the $z\sim 5.7$ Lyman-break galaxy HZ10 with the JWST/NIRSpec integral field unit in high and low spectral resolution (G395H, spectral resolving power $R\sim 2700$ and PRISM, $R\sim 100$, respectively), as part of the GA-NIFS programme. By spatially resolving the source (spatial resolution $\sim 0.15{^{\prime \prime }}$ or $\sim 0.9$ 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 H$\beta$, [O iii] $\lambda \lambda 4959{,}5007$, [N ii] $\lambda \lambda 6548{,}6584$, H$\alpha$, and [S ii] $\lambda \lambda 6716{,}6731$. The R100 data additionally contain a strong detection of the Ly $\alpha$ break, rest-frame UV and optical continuum, and [O ii] $\lambda \lambda 3726{,}3729$. 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 $\left(\rm \log _{10}\left({\it n}_e / cm^{-3}\right)\sim 3\right)$ and metallicity ($\sim 0.5-0.7$ 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 ($\beta _{\rm UV}\sim -0.9$) and significant H $\alpha$ emission, suggesting an evolved population and active star formation. A comparison to high-resolution ($\sim 0.3,{\rm {arcsec}}$ or $\sim 1.8$ kpc) [${\rm C, {\small II}}$] $158, \mu$m 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