RIOJA. A Clumpy Galaxy Assembly at Redshift 6.81 Revealed by JWST

Abstract Spatially resolved multiwavelength analysis is essential to study galaxy formation and evolution. A UV-bright galaxy COS-2987030247 at z = 6.81 is one of the Rosetta Stones in the epoch of reionization for which JWST NIRSpec Integral Field Spectroscopy, NIRCam imaging, and Atacama Large Millimeter/submillimeter Array data are available thanks to the RIOJA program. We identified the rest-frame optical emission lines from the ionized hydrogen, oxygen, and neon gas. The [O iii ] 5008 Å line emission and the NIRCam images show a complex kinematical and morphological structure where two bright main and three faint clumps are identified in a 10 kpc extent. The system is not classified as a purely rotation-dominated disk. The multiple clumps are instead consistent with a merger-related origin, including either distinct galaxies in interaction or star-forming clumps formed through tidal gas compression during a merger. The spatially resolved emission line fluxes show that dust attenuation, metal enrichment, and ionization parameter are preferentially enhanced in the star formation peaks. Our spectral energy distribution fitting suggests that the main clumps are in a moderately dust-attenuated star-forming phase ( A V = 0.2–0.3 and SFR(H α ) ∼10 M ⊙ yr −1 ) with almost zero escape fraction of ionizing photons. In contrast, the subclumps are dust-free and lying on or below the main sequence of star-forming galaxies. These subclumps may work as a perturber that triggers the clumpy starburst in the surrounding gas through the merger event.