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

Spatially resolved multi-wavelength 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 ALMA 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 \OIII,5008Å 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 SED fitting suggests that the main clumps are in a moderately dust-attenuated star forming phase ($A_{\rm V} = 0.2$–$0.3$ and SFR(H$α$) $\sim 10$,M$_\odot$,yr$^{-1}$) with almost zero escape fraction of ionizing photons. In contrast, the sub-clumps are dust-free and lying on or below the main sequence of star-forming galaxies. These sub-clumps may work as a perturber that triggers the clumpy starburst in the surrounding gas through the merger event.

💡 Research Summary

This paper presents a comprehensive, spatially‑resolved study of the UV‑bright galaxy COS‑2987030247 (hereafter COS‑2987) at redshift z = 6.81, using the synergy of JWST NIRCam imaging, JWST NIRSpec integral‑field spectroscopy (IFS), and ALMA sub‑millimetre observations. The RIOJA program (Reionization and the ISM/Stellar Origins with JWST and ALMA) provided deep NIRCam data in six broad‑band and two medium‑band filters (F115W to F444W) with a final pixel scale of 0.03″ and point‑spread‑function (PSF) full‑width at half‑maximum (FWHM) ranging from 0.06″ to 0.16″. These images reveal that COS‑2987 is not a smooth monolithic object but consists of five distinct clumps spread over ~10 kpc: two bright “main” clumps (north and south) and three fainter “sub‑clumps” (sN1, sN2, sS). The main clumps are each ~2 kpc in size, dominate the rest‑frame optical line emission, and host the bulk of the star formation. The sub‑clumps are smaller (0.5–1 kpc), show little line emission, and appear essentially dust‑free.

NIRSpec IFS observations were carried out with the G395H/F290LP configuration, delivering R ≈ 2700 (Δv ≈ 110 km s⁻¹) over 2.87–5.27 µm. The data cube, sampled at 0.05″ spatial pixels, provides maps of