Clump-Scale Dust Attenuation in Epoch of Reionization Galaxies: Spatially Resolved Properties from FirstLight Simulations

Understanding dust attenuation in galaxies at both integrated and spatially resolved scales is fundamental for accurately determining the physical properties of galaxies. Recent high-spatial-resolution observations with ALMA and JWST enable investigations of spatially resolved properties in high-redshift galaxies ($z \gtrsim 6$), but spatial variations in dust properties remain poorly constrained. We use cosmological zoom-in simulations combined with post-processing dust radiative transfer calculations for 376 clumpy galaxies at $z=6$-$9$ with stellar masses of $M_* \gtrsim 10^9 , M_\odot$. For each system, we investigate dust attenuation and re-emission properties for three components: system-integrated, individual clumps, and diffuse regions. We find that system-integrated attenuation curves are grayer than the Calzetti curve, even when assuming MW- or SMC-type dust. Attenuation curves of individual clumps are even grayer, while diffuse regions exhibit steeper curves owing to enhanced scattering in optically thin environments. Since the effects of optical depth and dust-star geometry are intrinsically degenerate in attenuation curves, we introduce a toy model based on the IRX-$Δβ$ plane, where $Δβ$ denotes the difference between attenuated and intrinsic UV slopes. Applying this framework, we find that clumps have dust column densities approximately an order of magnitude higher than system-integrated values and exhibit co-spatial or dust-extended geometries. In contrast, system-integrated attenuation reflects star-extended geometries driven by contributions from optically thin diffuse regions. We apply this framework to REBELS-IFU galaxies at $z \sim 7$ and find good agreement with our simulation predictions.

💡 Research Summary

This paper investigates spatial variations of dust attenuation and re‑emission in high‑redshift (z ≈ 6–9) clumpy galaxies using the FirstLight cosmological zoom‑in simulation suite combined with post‑processing dust radiative transfer (RT) calculations performed with the 3‑D Monte Carlo code SKIRT. The authors select 376 massive (M★ ≳ 10⁹ M⊙) systems containing 1,059 individual star‑forming clumps (size ≳ 100 pc, SFR > 1 M⊙ yr⁻¹) and analyze three distinct components for each galaxy: the integrated whole, the identified clumps, and the diffuse inter‑clump regions. Stellar emission is modeled with BPASS single‑stellar‑population spectra (Chabrier IMF, upper mass 300 M⊙) and the dust distribution is assumed to trace metals with a fixed dust‑to‑metal ratio DTM = 0.4. Two dust grain models (Milky Way and SMC) are employed, and the RT includes absorption, anisotropic scattering, self‑absorption, CMB heating, and non‑LTE emission from small grains and PAHs.

Key findings are: (1) Integrated attenuation curves are greyer (flatter) than the canonical Calzetti law, even when using MW‑type dust, indicating high effective optical depths and mixed star‑dust geometries. (2) Individual clumps exhibit even greyer curves, reflecting dust column densities roughly an order of magnitude larger than the galaxy‑average. (3) Diffuse regions, being optically thin, show steeper attenuation curves due to enhanced scattering. (4) To disentangle the degeneracy between optical depth and geometry, the authors develop a toy model on the IRX–Δβ plane, where Δβ = β_obs − β_int. The model parameterizes dust optical depth (τ) and the ratio of dust‑to‑star scale heights (h_dust/h_star). By fitting simulated data, they find clumps occupy a regime of high τ and h_dust/h_star ≈ 1–1.5 (co‑spatial or modestly dust‑extended), whereas the integrated galaxy lies at lower τ but with h_dust/h_star ≫ 1 (star‑extended geometry dominated by diffuse regions). (5) Applying the same analysis to REBELS‑IFU observations of z ≈ 7 galaxies yields dust column densities and geometry parameters consistent with the simulation predictions, providing an empirical validation of the model.

The paper emphasizes that assuming a uniform attenuation law across an entire galaxy can lead to systematic biases in derived stellar masses, ages, and star‑formation rates, especially when clumps contribute disproportionately to the UV and IR output. The IRX–Δβ toy framework offers a practical tool for interpreting limited observational data (IRX and UV slope) to infer internal dust‑star configurations without full spatially resolved SED fitting. Limitations include the fixed DTM value, omission of nebular continuum emission (justified by the typical ages of identified clumps), and uncertainties in high‑z dust grain composition. The authors suggest future work incorporating variable DTM, nebular contributions, and more diverse dust models.

Overall, the study provides a robust theoretical baseline for the upcoming wave of high‑resolution JWST and ALMA observations, demonstrating that clump‑scale dust properties can differ markedly from galaxy‑averaged values and that these differences are observable through combined IRX and UV‑slope diagnostics. This advances our understanding of dust physics during the epoch of reionization and informs more accurate interpretations of early galaxy evolution.