GA-NIFS: JWST/NIRSpec integral field unit observations of HFLS3 reveal a dense galaxy group at z 6.3

Massive, starbursting galaxies in the early Universe represent some of the most extreme objects in the study of galaxy evolution. One such source is HFLS3 (z similar to 6.34), which was originally identified as an extreme starburst galaxy with mild gravitational magnification (mu similar to 2.2). Here, we present new observations of HFLS3 with the JWST/NIRSpec integral field unit in both low (PRISM/CLEAR; R similar to 100) and high spectral resolution (G395H/290LP; R similar to 2700), with high spatial resolution (similar to 0.1') and sensitivity. Using a combination of the NIRSpec data and a new lensing model with accurate spectroscopic redshifts, we find that the 3''x 3'' field is crowded, with a lensed arc (C, z = 6.3425 +/- 0.0002), two galaxies to the south (S1 and S2, z = 6.3592 +/- 0.0001), two galaxies to the west (W1, z = 6.3550 +/- 0.0001; W2, z = 6.3628 +/- 0.0001), and two low-redshift interlopers (G1, z = 3.4806 +/- 0.0001; G2, z = 2.00 +/- 0.01). We present spectral fits and morpho-kinematic maps for each bright emission line (e.g. [OIII]lambda 5007, H alpha, and [NII]lambda 6584) from the R2700 data for all sources except G2 (whose spectral lines fall outside the observed wavelengths of the R2700 data). From a line ratio analysis, we find that the galaxies in component C are likely powered by star formation, though we cannot rule out or confirm the presence of active galactic nuclei in the other high-redshift sources. We performed gravitational lens modelling, finding evidence for a two-source composition of the lensed central object and a magnification factor (mu = 2.1 - 2.4) comparable to findings of previous work. The projected distances and velocity offsets of each galaxy suggest that they will merge within the next similar to 1 Gyr. Finally, we examined the dust extinction-corrected SFRH alpha of each z > 6 source, finding that the total star formation (510 +/- 140 M(circle star)yr-1, magnification-corrected) is distributed across the six z similar to 6.34 - 6.36 objects over a region of diameter similar to 11 kpc. Altogether, this suggests that HFLS3 is not a single starburst galaxy, but instead a merging system of star-forming galaxies in the epoch of reionisation.