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

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.

• The paper shows the discovery of a z=7.212 galaxy undergoing a very young starburst with a mass-weighted age of ∼1.8 Myr and an SFR of around 165 M⊙/yr.
• The paper employs JWST and ALMA data to spatially map high-velocity ionized gas outflows, detecting a broad [O III] component with velocities of approximately 535 km s⁻¹.
• The paper links extreme ISM conditions and rapid gas depletion to merger-driven assembly, suggesting these galaxies may evolve into massive quiescent systems at lower redshifts.

Young Starburst and Ionized Gas Outflows in a $z=7.212$ Galaxy: Insights from JWST NIRCam and NIRSpec Observations

Introduction

This study presents a comprehensive analysis of the $z=7.212$ galaxy SXDF-NB1006-2, leveraging JWST NIRCam imaging, NIRSpec IFU spectroscopy, and ALMA data as part of the RIOJA project. The work provides a detailed characterization of the galaxy's stellar populations, ISM conditions, ionized gas outflows, and evolutionary context within the reionization epoch. The analysis combines SED fitting, emission line diagnostics, spatial mapping, and comparative studies with other high-redshift galaxies, yielding new constraints on early starburst activity, feedback, and mass assembly.

Morphology and Photometry

NIRCam imaging reveals that the UV continuum of SXDF-NB1006-2 is elongated and clumpy, with a pronounced tail-like structure, suggestive of an edge-on disk or chain galaxy morphology. The optical emission is more extended and diffuse, likely due to a combination of lower spatial resolution at longer wavelengths and the presence of an ionized halo.

Figure 1: NIRCam images of SXDF-NB1006-2, showing the elongated, clumpy UV continuum and more extended optical emission.

Aperture photometry across six NIRCam filters ($1-5\,\mu$m) was performed using curve-of-growth analysis to capture the total flux, accounting for the extended emission. The F444W band, dominated by strong [O III] emission, exhibits the most extended morphology.

SED Fitting and Stellar Populations

The SED fitting, performed with BAGPIPES, incorporates NIRCam photometry, ALMA dust continuum non-detections, and [O III] line fluxes. Multiple SFH models were tested, with the lognormal SFH adopted as fiducial. The results indicate:

• A very young, bursty star-forming population with a mass-weighted mean age of $\sim$1.8 Myr.
• High instantaneous SFR ($\sim$165 $M_\odot$ yr$^{-1}$), placing the galaxy well above the main sequence at this redshift and mass.
• Low dust attenuation ($A_V \sim 0.2$ mag), consistent with the non-detection of dust continuum.
• Gas-phase metallicity of $Z \sim 0.2\,Z_\odot$.
• High ionization parameter ($\log U \sim -1.6$), indicative of a hard radiation field from massive young stars.

The SED exhibits a prominent Balmer jump, consistent with strong nebular continuum and a very young stellar population.

Figure 2: SED fit for SXDF-NB1006-2 with a lognormal SFH, showing the observed photometry, model fit, and predicted Balmer jump.

Emission Line Analysis and Outflow Diagnostics

NIRSpec IFU spectroscopy detects multiple nebular lines, including [O II], [Ne III], H$\gamma$, H$\beta$, and [O III] $\lambda$4960, 5008. The [O III] $\lambda$5008 line exhibits a robust broad component (FWHM $\sim$630 km s$^{-1}$), unambiguously indicating the presence of high-velocity ionized gas outflows.

Figure 3: NIRSpec IFU spectra showing detected emission lines and the broad [O III] $\lambda$5008 component.

Double Gaussian fitting and residual analysis confirm the broad component's significance. The outflow velocity is estimated at $535 \pm 132$ km s$^{-1}$, exceeding the escape velocity of the host halo, though only a fraction of the outflowing material is likely to escape.

Figure 4: Double Gaussian decomposition of [O III] $\lambda$5008, with residuals demonstrating the necessity of a broad component.

Spatial mapping of the narrow and broad [O III] emission reveals that the outflowing gas is more extended ($r_e \sim 1.9$ kpc) than the star-forming regions traced by the UV continuum and narrow [O III].

Figure 5: Moment-0 maps of the narrow and broad [O III] $\lambda$5008 components, highlighting the spatial extent of the outflow.

The mass outflow rate and mass loading factor are high, with the total (all phases) mass loading factor potentially exceeding unity, though uncertainties remain due to assumptions about the neutral and molecular outflow phases.

ISM Conditions and Metallicity

Strong-line diagnostics yield a gas-phase metallicity of $12+\log(\mathrm{O/H}) \sim 8.07$ ($Z \sim 0.2\,Z_\odot$), consistent with SED-based estimates. The galaxy exhibits extreme ISM conditions:

• O32 ratio ($\sim$18) and Ne3O2 ratio ($\sim$0.06) at the upper end of $z>6$ samples.
• Rest-frame EW([O III]+H$\beta$) $\sim$3700 Å, among the highest at this epoch.
• Ionizing photon production efficiency $\log(\xi_\mathrm{ion}) \sim 25.44$.

These properties place SXDF-NB1006-2 among the most extreme starbursts known at $z>6$.

Gas Mass, Depletion Time, and Evolutionary Implications

ALMA [C II] data provide constraints on the molecular and atomic gas masses, yielding a total gas mass of $\sim 2 \times 10^{10}\,M_\odot$ and a gas fraction of $\sim$98%. The gas depletion time is short ($\sim$114–445 Myr), implying rapid consumption of the gas reservoir and suggesting that SXDF-NB1006-2 could be a progenitor of massive quiescent galaxies observed at $z\sim4-5$.

Spatial Structure and Merger Signatures

Comparisons of the spatial distribution of UV continuum, optical [O III], and FIR [O III] emission reveal:

• The southern clump of FIR [O III] aligns with the UV and optical [O III] peaks.
• The northern FIR [O III] clump is offset, possibly indicating inhomogeneous ISM density or unresolved star-forming regions.
• The clumpy, elongated, and tail-like morphology, along with high merger and clumpy fractions among ALMA-detected galaxies, supports a scenario of recent or ongoing merger-driven starburst activity.

  Figure 6: Left: UV continuum and FIR [O III] spatial comparison. Right: Optical and FIR [O III] emission overlay, highlighting morphological differences.

Underlying Old Stellar Populations

Despite the extremely young mass-weighted age, the observed metallicity requires prior star formation episodes. SED modeling with non-parametric SFHs and constraints from the F356W band suggest a possible underlying old stellar population, with an upper limit on the old stellar mass of $\log(M_*^\mathrm{old}/M_\odot) < 9.5$. Future JWST/MIRI F770W observations are predicted to be sensitive enough to confirm or rule out the presence of such populations.

Comparative Context: Mass Assembly and Metallicity Relations

A comparative analysis with other $z>6$ galaxies shows that ALMA-detected galaxies are systematically more massive, younger, and UV-brighter than JWST-selected samples, and exhibit a steeper mass-metallicity relation.

Figure 7: Age–mass relation for ALMA-detected and JWST-selected galaxies at $z>6$, color-coded by $M_\mathrm{UV}$.

Figure 8: Mass–metallicity relation for ALMA-detected and JWST-selected galaxies, with the target highlighted.

This suggests that ALMA-detected galaxies have undergone more efficient and rapid mass assembly, likely driven by frequent mergers and intense starburst episodes.

Conclusion

This work provides a detailed physical characterization of a $z=7.212$ starburst galaxy, revealing:

• A dominant, extremely young starburst population with high SFR and ionization parameter.
• Robust detection of high-velocity ionized gas outflows, with spatially extended broad [O III] emission.
• ISM conditions and metallicity requiring prior star formation, despite the young observed age.
• Morphological and kinematic evidence for recent merger-driven assembly.
• A short gas depletion time, positioning the galaxy as a plausible progenitor of massive quiescent systems at lower redshift.
• ALMA-detected galaxies at $z>6$ are distinct in their mass, age, and metallicity scaling relations, reflecting more efficient early mass assembly.

The results underscore the power of joint JWST and ALMA observations for dissecting the physical processes governing early galaxy evolution. Future MIRI observations will be critical for constraining the presence of old stellar populations and refining the evolutionary pathways of massive galaxies in the reionization era.