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 IFU 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 program. 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$\beta$, [OIII]$\lambda \lambda 4959,5007$, [NII]$\lambda \lambda 6548,6584$, H$\alpha$, and [SII]$\lambda \lambda 6716,6731$. The R100 data additionally contains a strong detection of the Ly$\alpha$ break, rest-frame UV and optical continuum, and [OII]$\lambda \lambda 3726,3729$. 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 $\left({\log }_{10}\left({\mathrm{n}}_{\mathrm{e}}/c{\mathrm{m}}^{-3}\right)\sim 3\right)$ 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 (${\beta }_{\mathrm{UV}}\sim -0.9$) and significant H$\alpha$ emission, suggesting an evolved population and active star formation. A comparison to high resolution ($\sim 0.3^{\prime \prime }$ or $\sim 1.8$kpc) [CII]$158\mu$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.

19 pages, 11 figures. Accepted for publication in MNRAS