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

TL;DR

This study uses JWST/NIRSpec IFS, JWST/NIRCam imaging, and ALMA data to dissect COS-2987030247, a clumpy galaxy at z = 6.81, revealing a complex, merger-like assembly rather than a simple rotation-dominated disk. The analysis identifies five [O III] 5008 Å-bright clumps with varied metallicities, dust content, and stellar populations; spatially resolved line diagnostics show elevated dust, metal enrichment, and ionization at star-forming peaks, while SED modeling indicates very young, modest-dust main clumps and older populations in sub-clumps. The results favor a major-merger scenario with tidal features driving clump formation and a multi-phase ionized/neutral ISM, offering an integrated view of early galaxy assembly and reionization-era ISM conditions. The work demonstrates the power of JWST and ALMA synergy to resolve ISM, kinematics, and stellar populations in the first billion years of cosmic history, with implications for the ionizing budget and the interpretation of high-z galaxy morphologies.

Abstract

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.

Figures (14)

• Figure 1: (Top panel) One-dimensional spectrum of COS-2987 extracted from the NIRSpec IFS data cube. Vertical dashed lines indicate the detected emission lines. (Right panel) The aperture used for extraction of the spectrum is shown by a dashed ellipse which is superposed on the F115W (blue) $+$ [O iii] 5008Å (green) $+$ F200W (red) pseude color image. These three images were smoothed so that their PSFs are matched (section \ref{['sec:MultiPhot']}). Contours show the [O iii] 5008Å integrated intensity map at $2 \times$ and $10 \times \sigma$. Panel size is $2" \times 2"$ corresponding to 11.6 kpc $\times$ 11.6 kpc at $z = 6.81$. (Bottom panels) Zoom-in spectra of the detected emission lines, where the horizontal axis is the velocity relative to the best-fit redshift to [O iii] 5008Å, $z = 6.80731$. In each panel, the black solid histogram and gray line indicate the observed and $1\sigma$ error spectra, respectively. The red solid line and red shade show the best-fit Gaussian profile and $68$ % confidential interval from the MC iteration (see text). For the [O ii] 3727,3730Å doublet, the best-fit models for individual lines are shown by the red dashed lines.
• Figure 2: The [O iii] 5008Å line integrated intensity (left), velocity (middle), and velocity dispersion (right) maps. Each panel has $2\hbox{$.\!\!^{\prime\prime}$}2 \times 2\hbox{$.\!\!^{\prime\prime}$}2$ in size. Ellipses correspond to the photometric apertures for the multiple clumps (section \ref{['sec:MultiPhot']}). Contours in the integrated intensity map are $-2$ (dashed), 2, 4, 8, 16, and $32 \times \sigma$ (solid) of the line brightness. Contours in the velocity and velocity dispersion maps show the F115W surface brightness profile before the PSF matching at $-2$ (dashed), 2, and $4 \times \sigma$ (solid) levels. The F115W surface brightness is sensitive to 100 Myr time-scale star formation. The rightmost panel is also the velocity dispersion map but zoomed around the highest velocity dispersion region between mN and sN1. We also superposed the observed (blue), error (grey dashed) spectra, and the best-fit Gaussian profile (orange) on each pixel.
• Figure 3: The [O iii] 5008Å line profiles of the five clumps. The velocity axis is centered at $z = 6.80731$. In each panel, the black solid histogram and gray line indicate the observed and $1\sigma$ error spectra, respectively. The red solid line and red shade show the best-fit Gaussian profile and $68$ % confidential interval from the MC iteration. The blue vertical dashed line and shade indicate the best-fit peak velocity and uncertainty of each clump.
• Figure 4: Multiple clumps identified in the NIRSpec line integrated intensity maps and NIRCam images. We also show the [O iii] 5008Å velocity and ALMA [O iii] 88$\mu$m integrated intensity maps at the top left panels. Not only the original data but also the PSF-matched data are shown in the left- and right-side panels, respectively. Each panel has $2\hbox{$.\!\!^{\prime\prime}$}2 \times 2\hbox{$.\!\!^{\prime\prime}$}2$ in size. In the all panels, the photometric apertures for the multiple clumps are shown by magenta ellipses. Contour levels are $-2$ (dashed), 2, 4, 8, 16, and $32 \times \sigma$ (solid). The low-$z$ contaminant, L17z2, is masked when generating the PSF-matched NIRCam images (see Section \ref{['sec:MultiPhotClump']}).
• Figure 5: Maps of physical quantities: dust attenuation $A_{\rm V}$, 10 Myr time-scale (H$\alpha$) star formation rate surface density $\Sigma_{\rm SFR}$, metallicity $12 + \log({\rm O/H})$, ionizing parameter $\log(U_{\rm ion})$, and electron density $\log(n_{\rm e}/{\rm cm}^{-3})$ from left to right. Pixel-to-pixel physical properties of $A_{\rm V}$, SFR, $12+\log({\rm O/H})$, $\log(U_{\rm ion})$, and $\log(n_{\rm e})$ are derived from Balmer decrement (H$\alpha$/H$\beta$), H$\alpha$, R3 index ([O iii] 5008Å, H$\beta$), O32 index ([O iii]5008Å, [O ii] doublet), and [O ii] 3727,3730Å line ratio, respectively. Each panel covers 6.9 $\times$ 6.9 kpc at $z = 6.81$. The multiple clumps in COS-2987 are shown by ellipses. Contours show the PSF-matched F115W surface brightness profile at 2, 4, 8, and 16 $\times \sigma$ levels. The F115W surface brightness is sensitive to $100$ Myr time-scale star formation.