RIOJA. Young Starburst and Ionized Gas Outflows in a $z = 7.212$ Galaxy Uncovered by JWST NIRCam and NIRSpec Observations

TL;DR

This study uses JWST NIRCam/NIRSpec and ALMA observations to dissect a $z=7.212$ galaxy SXDF-NB1006-2, revealing a young ($\sim2$ Myr) starburst with a high ionization field and a robust broad [O III] outflow ($v_{out}\sim$535 km s$^{-1}$). The galaxy is extremely gas-rich ($M_{gas}\sim1.9\times10^{10}\,M_\odot$) with a short gas depletion time ($t_{depl}\sim114$–$445$ Myr) and may be a progenitor of massive quiescent galaxies at $z\sim4-5$, while signs point to potential old stellar populations up to $\log(M_*^{old}/M_\odot)\lesssim9.5$. Spatially, UV, optical [O III], and FIR [O III] 88 μm emissions trace clumpy, anisotropic structures likely shaped by density inhomogeneities and past mergers. By comparing ALMA-detected and JWST-selected high-$z$ galaxies, the work suggests ALMA-detected systems are typically more massive and younger, with a steeper mass–metallicity relation, reflecting more efficient early mass assembly. The results underscore the power of combining JWST and ALMA to illuminate ISM conditions, feedback, and stellar assembly in the reionization era, with MIRI follow-up proposed to test for ancient stellar populations.

Abstract

We present analysis of 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 (RIOJA) project. We derive the physical properties by conducting spectral energy distribution (SED) fitting, revealing that our target is a young (age $\sim2$ Myr) starburst galaxy with intense radiation field. We detect multiple nebular emission lines from NIRSpec IFS data. We identify a robust broad component of [O III]$\lambda5008$ 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 $z\sim4-5$ identified by recent JWST observations. The spatial distribution of optical and far-infrared (FIR) [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 $\sim0.2\,\rm{Z}_\odot$, 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.

Figures (11)

• Figure 1: NIRCam images of SXDF-NB1006-2. The black circle in the bottom right corner of each panel indicates the PSF FWHM taken from Finkelstein. Above the dashed line in F115W represents the tail structure.
• Figure 2: SED fit for SXDF-NB1006-2 with Lognormal SFH model. The horizontal axis shows the rest-frame wavelength. The light orange line represents the 50th percentile of the posterior spectrum output from the SED fitting, and the orange shaded region shows its $1\,\sigma$ deviation (16th-84th percentiles). The dark orange empty squares are the observational data and the brown empty circles are the model photometry outputs from SED fitting. The horizontal error bars of the observational data points represent the wavelength coverage of the filters. Left panel illustrates fitting in rest-UV and optical range with JWST measurements. Right panel shows results in rest-FIR range with ALMA measurements.
• Figure 3: Spectra extracted from NIRSpec IFU data. Top panel: full spectrum obtained from a circular aperture with a radius of $0\farcs5$, with key emission and absorption lines labeled. The light grey shaded region indicates $1\,\sigma$ noise level. Middle and bottom panels: spectra of detected emission lines extracted using different aperture sizes to measure their total fluxes. Continuum levels were measured in corresponding aperture sizes and subtracted. From left to right, the bottom panel shows H$\beta$ and [O iii]$\lambda$$\lambda$4960, 5008 lines, while the middle panel displays [O ii]$\lambda$$\lambda$3727, 3730, [Ne iii]$\lambda$3870, He i$\lambda$3966+[Ne iii]$\lambda$3969+H$\epsilon$ blended and H$\gamma$ lines. The radii of circular apertures from which the spectra were extracted to measure total fluxes are shown in the labels. The red dotted line corresponds to the noise spectra measured from random aperture photometry using the same aperture sizes as the spectral extractions. Black vertical dashed lines demarcate the wavelength ranges for which the spectra were generated with different aperture sizes. Brown solid lines indicate single Gaussian fits to the emission lines, and grey shaded regions highlight integration ranges for each line.
• Figure 4: Moment 0 maps of detected lines using NIRSpec IFU data. The dashed black and white circles indicate the circular apertures from which we extracted the spectra to measure total fluxes. In upper panels, the grey contours indicate [1, 2, 3]$\sigma$ significance levels, where $1\,\sigma$ noise level is the standard deviation measured after $3\,\sigma$ clipping over the entire FoV on each moment-0 map, which is different from the method we used for measuring the uncertainties of line fluxes and actual S/Ns. Similarly, in bottom panels, the grey and white contours show [1, 2, 5, 7]$\sigma$, [1, 2, 5, 10, 13]$\sigma$ and [1, 2, 5, 10, 20, 28]$\sigma$ significance levels of H$\beta$ and [O iii] doublet lines, respectively.
• Figure 5: Double Gaussian fitting results and residual analysis for [O iii]$\lambda$$\lambda$4960, 5008 lines. In the top panel, the solid grey line shows the spectrum extracted from the circular aperture to measure the total flux of each line. The grey vertical dashed line demarcates the wavelength ranges for which the spectra were generated with different aperture sizes. The dashed pink line shows the noise spectrum obtained from random aperture photometry with same aperture size to measure the total flux of the corresponding line. The dotted dashed green line shows the narrow component, the dotted blue line shows the broad component and the solid red line shows the sum of each line's narrow and broad components. The bottom panel shows the spectral residual after subtracting the single (orange dashed) or double (blue solid) Gaussian fitting curve.