2025MNRAS.544.4722R

We present the analysis of James Webb Space Telescope (JWST) NIRCam and NIRSpec observations of the galaxy SXDF-NB1006-2 at $z = 7.212$, as part of the Reionization and the ISM/Stellar Origins with JWST and ALMA project. We derive the physical properties by conducting spectral energy distribution fitting, revealing that our target is a young (age $\sim 2$ Myr) starburst galaxy with intense radiation field. We detect multiple nebular emission lines from NIRSpec integral field spectroscopy data. We identify a robust broad component of [O III] $\lambda$5008 emission, indicating the presence of ionized gas outflows. The derived gas depletion time of a few hundred Myr implies that our target could be one of the progenitors of massive quiescent galaxies at $z\sim 4\!-\!5$ identified by recent JWST observations. The spatial distribution of optical and far-infrared [O III] emission lines differs in morphology, likely resulting from different critical densities and inhomogeneous density distributions within the galaxy. Potential old stellar populations may be necessary to account for the derived metallicity of $\sim 0.2\, \rm {Z}_\odot$, and their presence can be confirmed by future MIRI observations. Including our target, star-forming galaxies at $z>6$ detected by ALMA are generally very young but more massive and brighter in UV than galaxies identified by only JWST. The ALMA-detected galaxies may also have a steeper mass-metallicity relation. These findings suggest that the ALMA-detected galaxies may have experienced more efficient mass assembly processes in their evolutionary pathways.