ELPIS: Accelerated metal and dust enrichment in a proto-cluster core at $z\approx8$

TL;DR

This study probes metal, dust, and molecular gas content in the A2744-z7p9OD proto-cluster at z ~ 7.88, focusing on the Quintet and Chain groups to understand early dust production and growth in a dense environment. By combining ALMA [C II] emission and 1.26 mm continuum with JWST-derived stellar masses and gas-phase metallicities, the authors derive molecular gas masses from the [C II] luminosity via a calibrated relation and dust masses from rest-frame 142 μm emission under an assumed dust temperature, revealing a transitional dust mass assembly phase. They find $\log(M_{\rm gas}/M_\odot) \sim 9.0$–$9.6$ and $\log(M_{\rm dust}/M_\odot) \sim 6.0$–$6.4$, with $\log(M_{\rm dust}/M_\star) \sim -3$ to $-2$ at $\log(M_{\star}/M_\odot) \sim 9$, and $\log(M_{\rm dust}/M_{\rm gas}) \sim -4$ to $-3$ at $12+\log(\mathrm{O/H}) \sim 8$. The Quintet shows higher metallicities and dust detectability than the Chain, implying environmental acceleration of enrichment and a transition from SN-driven dust production to ISM grain growth by these early epochs. Uncertainties in dust temperature and calibration of gas masses are discussed, but the results support a scenario where dense proto-cluster cores foster rapid chemical evolution and dust assembly at $z\gtrsim 8$.

Abstract

We present a study of the metal, dust, and molecular gas content in galaxies within the A2744-z7p9OD proto-cluster at z ~ 7.88. We focus on two galaxy groups, the Quintet and the Chain, which are covered by the ELPIS survey (The Emission-Line Protocluster Imaging Survey of the furthest overdensity beyond Pandora's Cluster Abell 2744). [C II] 158 um emission is detected in five galaxies, revealing molecular gas reservoirs with log(M_gas/Msun) ~ 9.0-9.6, while dust continuum at the observed frame of 1.26 mm is detected in three galaxies, yielding dust masses of log(M_dust/Msun) ~ 6.0-6.4, assuming a dust temperature of T_dust = 45 (+15, -15) K. The derived properties, including stellar-to-dust mass ratios of log(M_dust/M_star) ~ -3 to -2 at log(M_star/Msun) ~ 9, and dust-to-gas mass ratios of log(M_dust/M_gas) ~ -4 to -3 at 12+log(O/H) ~ 8, place these galaxies in an intermediate regime: higher than the very low ratios expected from supernova-driven dust production, but still below the levels attained once efficient grain growth dominates. These values indicate a transition phase of dust mass assembly, likely reflecting the onset of grain growth via metal accretion under accelerated evolution in the proto-cluster core.

Figures (4)

• Figure 1: The middle panel shows an HST+JWST color composite image (F814W/F115W/F444W) of a portion of the A2744-z7p9OD proto-cluster, with F814W assigned to blue, F115W to green, and F444W to red, at $z \approx 7.88$. Yellow contours indicate the stellar-mass-weighted density map constructed from spectroscopically confirmed member galaxies, corresponding to 55%, 75%, and 95% of the peak mass density 2025arXiv250706284W. The left and right panels show enlarged views of the Chain and Quintet groups, respectively, with overlaid 1.26 mm dust continuum (top) and [C ii] line (bottom) white contours starting at $\pm2\sigma$ and increasing in steps of $1\sigma$. Naturally weighted maps are used for the Quintet, while tapered images are shown for the Chain.
• Figure 2: Distribution of galaxies in the A2744-z7p9OD proto-cluster and other $z \sim 7$--8 samples 2024ApJ...975...87M2024ApJ...977L..36H2024AA...688A.146A2025arXiv250110508A2025arXiv250110559R in the stellar mass--metallicity plane. All metallicities are derived using strong-line calibrations. The mass--metallicity relation from 2024ApJ...971...43M is shown for comparison as a blue shaded region. YD1 and YD4$+$YD6 in the Quintet exhibit elevated gas-phase metallicities relative to the relation, while ZD3$+$ZD6 and ZD12 in the Chain lie on or below the relation.
• Figure 3: Comparison of dust mass ($M_{\rm dust}$) versus stellar mass ($M_\star$) for our sample and other $z \sim 7$--8 galaxies 2015Natur.519..327W2020MNRAS.493.4294B2021MNRAS.508L..58B2024ApJ...975...87M2025arXiv250110508A, along with model predictions at $z=7$2017MNRAS.471.3152P and at $z=6-12$2025ApJ...982....7N. The A2744-z7p9OD proto-cluster galaxies occupy the lower stellar mass regime and show relatively low dust-to-stellar mass ratios, $\log (M_{\rm dust}/M_\star)$, compared to the REBELS sample at $z\sim7$. Simulations predict that a rapid rise in this ratio occurs around $\log (M_\star/\mathrm{M}_\odot) \sim 9$ at $z \sim 8$, driven by efficient dust growth via metal accretion in the ISM. The galaxies in the Quintet and Chain may represent this transitional phase.
• Figure 4: Dust-to-gas mass ratio, $\log (M_{\rm dust}/M_{\rm gas})$, as a function of gas-phase metallicity for galaxies in our sample and comparison samples 2024ApJ...975...87M2024ApJ...977L..36H2025arXiv250110508A. The solid curve indicates the predicted relation from (semi-)analytical calculations of 2017MNRAS.471.3152P at $z=7$. YD1 and YD4+YD6 in the Quintet exhibit relatively high metallicities, comparable to those of the REBELS sample, suggesting that they may be undergoing rapid dust mass buildup driven by grain growth in the ISM.