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GA-NIFS: JWST/NIRSpec IFU observations of HFLS3 reveal a dense galaxy group at z~6.3 Published: 8/31/2023 1:28:16 PM Updated: 8/31/2023 1:28:16 PM

Paper abstract: Massive, starbursting galaxies in the early Universe represent some of themost extreme objects in the study of galaxy evolution. One such source is HFLS3(z~6.34), which was originally identified as an extreme starburst galaxy withmild gravitational magnification. Here, we present new observations of HFLS3with the JWST/NIRSpec IFU in both low (PRISM/CLEAR; R~100) and high spectralresolution (G395H/290LP; R~2700), with high spatial resolution (~0.1") andsensitivity. Thanks to the combination of the NIRSpec data and a new lensingmodel with accurate spectroscopic redshifts, we find that the 3"x3" field iscrowded, 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 andmorpho-kinematic maps for each bright emission line (e.g., [OIII]5007, Halpha,[NII]6584) from the R2700 data for all sources except G2. From a line ratioanalysis, the galaxies in C are likely powered by star formation, while wecannot rule out or confirm the presence of AGN in the other high-redshiftsources. We perform gravitational lens modelling, finding evidence for atwo-source composition of the lensed central object and a comparablemagnification factor (mu=2.1-2.4) to previous work. The projected distances andvelocity offsets of each galaxy suggest that they will merge within the next~1Gyr. Finally, we examine the dust extinction-corrected SFR of each z>6source, finding that the total star formation (460+/-90 Msol/yr,magnification-corrected) is distributed across the six z~6.34-6.36 objects overa region of diameter ~11kpc. Altogether, this suggests that HFLS3 is not asingle starburst galaxy, but instead is a merging system of star-forminggalaxies in the Epoch of Reionization.