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Extended enriched gas in a multi-galaxy merger at redshift 6.7

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Abstract

Recent JWST observations have uncovered high-redshift galaxies characterized by multiple star-forming clumps, many of which appear to be undergoing mergers. Such mergers, especially those of two galaxies with equivalent masses, play a critical role in driving galaxy evolution and regulating the chemical composition of their environments. Here we report a major merger of at least 5 galaxies, dubbed JWST’s Quintet (JQ), at redshift 6.7. This system resides in a small area of approximately 4.5″ × 4.5″ (24.6 × 24.6 pkpc2), containing over 17 galaxy-size clumps with a total stellar mass of 1010 M⊙. JQ has a total star formation rate of 255 M⊙ yr−1, placing it approximately 1 dex above the star formation rate–mass main sequence at this epoch. The high mass and star formation rate of JQ are consistent with the star formation history of those unexpected massive quiescent galaxies observed at redshift 4–5, offering a plausible evolutionary pathway for the formation of such galaxies. We also detect a large [O iii] + Hβ gaseous halo surrounding and connecting four galaxies in JQ, suggesting the existence of metals in the surrounding medium—the inner part of its circumgalactic medium. This provides direct evidence for metal enrichment of galaxies’ environments through merger-induced tidal stripping, just 800 Myr after the Big Bang.

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Fig. 1: The pseudo-colour images of JQ at redshift 6.71.
Fig. 2: Redshift measurements and SEDs of the ELGs in JQ.
Fig. 3: Distribution of the five galaxies, the associated clumps, and the entire JQ system in the star formation rate–stellar mass plane.
Fig. 4: The expected stellar mass growth of the JQ system.

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Data availability

The unprocessed JWST data are available through the Mikulski Archive for Space Telescopes (https://mast.stsci.edu/search/ui/#/jwst; PID 1180, 1181, 1210, 1286, 1895, 1963 and 3215). The reduced JWST images and spectra in this work are publicly available through the STSci High-Level Science Products (https://archive.stsci.edu/hlsp/jades; https://doi.org/10.17909/8tdj-8n28). The mock catalogues from the lightcone simulation are publicly available through the Semi-analytic forecasts for the Universe homepage (https://www.simonsfoundation.org/semi-analytic-forecasts).

Code availability

Codes used in this study are publicly available: Astropy (https://www.astropy.org), Bagpipes (https://github.com/ACCarnall/bagpipes), CIGALE (https://cigale.lam.fr), Pysersic (https://github.com/pysersic/pysersic), sep (https://github.com/sep-developers/sep), SExtractor (https://www.astromatic.net/software/sextractor, and MVT binning (https://github.com/pierrethx/MVT-binning).

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Acknowledgements

This research was supported in part by grant NSF PHY-2309135 to the Kavli Institute for Theoretical Physics (KITP). This work is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. This work was supported in part by generous funding provided by Marsha and Ralph Schilling through Texas A&M University.

Author information

Authors and Affiliations

  1. Department of Physics and Astronomy, Texas A&M University, College Station, TX, USA

    Weida Hu, Casey Papovich, Lu Shen, Justin Spilker & Justin Cole

  2. George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, USA

    Weida Hu, Casey Papovich, Lu Shen, Justin Spilker & Justin Cole

  3. Department of Physics, University of California, Santa Barbara, Santa Barbara, CA, USA

    Zixuan Peng

  4. Space Telescope Science Institute, Baltimore, MD, USA

    L. Y. Aaron Yung

  5. Gemini Observatory, Hilo, HI, USA

    Brian C. Lemaux

  6. Department of Physics and Astronomy, University of California Davis, Davis, CA, USA

    Brian C. Lemaux

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Contributions

W.H. and C.P. designed the layout of this paper. W.H. reduced the data, performed scientific analysis and wrote the paper. C.P. helped with the paper writing and scientific analysis. L.S. performed the spatially resolved analysis. C.P. and L.Y.A.Y. helped with the lightcone simulation and merging timescale estimation. Z.P. performed the outflow analysis based on analytical galactic wind models. B.C.L., J.S. and J.C. helped with the interpretation of the results. All authors discussed the results and commented on the paper.

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Correspondence to Weida Hu.

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Nature Astronomy thanks Takuya Hashimoto, Themiya Nanayakkara and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Illustration of clump identification.

a) The original F115W image of JQ. We adopt the F115W as the detection image because it has the best spatial resolution and is not affected by the Lyα breaks of the galaxies. b) The smoothed F115W image of JQ. We smooth the original F115W image by a two-dimensional Gaussian Kernel with σ = 2 pixel (0.06 arcsec). c) The contrast image, which is derived by subtracting the smoothed F115W image from the original F115W image. This step removes the outshining component and enhances the contrast of clump cores. We detect the clumps from this contrast image and mark the automatically-identified clumps as red circles. d) The F356W image of JQ. We notice several clumps that are only marginally detected in F115W but are clearly visible in F356W (green circles). Therefore, we manually add them to the catalog. e) The best-fit Sèrsic models of all galaxies in the image. This includes all galaxies and clumps. f) The residual map after subtracting the best-fit Sèrsic models for galaxies and the clumps from the original F115W image. The lack of large structures indicates we have well-modeled the light in these galaxies and their clumps.

Extended Data Fig. 2 Size – SFR relation and size – mass relation of the JQ clumps.

a) The size – SFR relation. The red dots indicate the individual clumps in the JQ system. The error bars correspond to the 1σ confidence intervals of the posterior distributions from the SED fitting and morphological fitting. For comparison, we compile several objects from the lensing galaxies and field galaxies at similar redshifts. The blue open triangles represent the field galaxies at z ~ 5 – 149. The purple open diamonds, squares, and pentagons represent the star clusters and star-forming clumps identified within the individual lensing galaxies at z > 66,7,8. The error bars correspond to the 1σ confidence intervals. The dashed lines indicate the star formation rate surface densities of 104, 103, 102, 101, 100, and 10−1M⊙kpc−2 from left to right. b) The size – mass relation. We present the JQ clumps as the red dots. The purple triangles and pentagons represent the star clusters and star-forming clumps identified within the individual lensing galaxies at z > 58,72. The error bars correspond to the 1σ confidence intervals. The blue and orange solid lines indicate the best-fit size – mass relation for 5≤z < 6 and 6≤z < 9 galaxies73, and the shaded regions represent their 1σ scatter. The dashed lines represent the extrapolation of these relations.

Extended Data Fig. 3 Expected distribution of clump numbers in galaxy systems in the JADES survey.

We select the galaxy systems with sizes similar to the JQ from a 2-deg2 cosmological simulation16 and rescale them to match the survey volume like JADES. The black solid curve indicates the cumulative fraction of systems as a function of galaxy counts. The black dashed line marks the selection criterion of the JQ-like system in the lightcone.

Extended Data Fig. 4 Distribution of all the JQ clumps, and the entire JQ system in the star formation rate – stellar mass plane.

The red squares and red dots represent the best-fit values from the SED fitting, with the error bars showing the 16th and 84th percentiles of the distributions of 400 posteriors. The clump C16 with a very small SFR of \(0.00{5}_{-0.005}^{+0.057}{M}_{\odot }{{\rm{yr}}}^{-1}\) is marked by a red triangle. We also plot a series of measurements at similar redshifts from the literature and the best-fit SFMS relations based on those samples17,18,19,20. The error bars represent the 1σ confidence intervals. In all cases, the clumps in JQ with stellar masses \(\log {M}_{\star }/{M}_{\odot }\lesssim 9\) have SFRs more than 1 dex above the measured SFMS relation of ref. 17, indicating the clumps are in a bursting phase. The total SFR of galaxies in the JQ system is higher than the SFMS by ~ 1 dex and among the top SFR of the galaxies at similar redshifts.

Extended Data Fig. 5 Illustration of generating the [O III]+Hβ map.

a) The F356W image after removing the foreground galaxies. b) The continuum model generated based on the best-fit SEDs of clumps, the Sèrsic profiles of clumps, and the F356W PSF. c) The [O III]+Hβ map derived by subtracting the continuum model from the foreground-removed F356W image. The green contour indicates surface brightnesses of \(8.8\times 1{0}^{-17}{\rm{erg}}\,{{\rm{s}}}^{-1}\,{{\rm{cm}}}^{-2}{{\rm{arcsec}}}^{-2}\), the same as Fig. 1.

Extended Data Table 1 Physical properties of the 17 galaxy-size clumps and the entire JQ system
Full size table

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Supplementary Information

Supplementary Figs. 1–3 and Discussion.

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Hu, W., Papovich, C., Shen, L. et al. Extended enriched gas in a multi-galaxy merger at redshift 6.7. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02636-1

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