Dust-UV offsets in high-redshift galaxies in the Cosmic Dawn III simulation

¹ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
² Institut d’Astrophysique de Paris, UMR 7095, CNRS, UPMC Univ. Paris VI, 98 bis boulevard Arago, 75014 Paris, France
³ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁴ School of Physics and Astronomy, The University of Nottingham, University Park, Nottingham, NG7 2RD UK
⁵ Univ. Lille, CNRS, Centrale Lille, UMR 9189 CRIStAL, F-59000 Lille, France
⁶ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
⁷ Leibniz-Institut für Astrophysik, An der Sternwarte 16, 14482 Potsdam, Germany
⁸ Department of Astronomy, Texas Center for Cosmology and Astroparticle Physics, Cosmic Frontier Center, The University of Texas at Austin, Austin, TX, 78712 USA
⁹ Department of Physics, California Polytechnic State University, San Luis Obispo, CA, 93407 USA
¹⁰ Department of Astrophysical Sciences, Princeton University, 4 Ivy Ln, Princeton, NJ, 08544 USA
¹¹ Departamento de Física Teórica M-8, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
¹² Centro de Investigación Avanzada en Física Fundamental (CIAFF), Universidad Autónoma de Madrid, 28049 Madrid, Spain
¹³ Astronomy Center, Department of Physics & Astronomy, Pevensey II Building, University of Sussex, Falmer, Brighton, BN1 9QH United Kingdom
¹⁴ Chosun University, 375 Seosuk-dong, Dong-gu, Gwangjiu, 501-759 Korea
¹⁵ Lawrence Berkeley National Laboratory, CA, 94720-8139 USA
¹⁶ Berkeley Center for Cosmological Physics, UC Berkeley, CA, 94720 USA

^⋆ Corresponding author: pierre.ocvirk@astro.unistra.fr.

Received: 3 September 2024
Accepted: 5 September 2025

Abstract

Recent observations reveal puzzling spatial offsets between the ALMA dust continuum from bright galaxies at z = 5–7 and their UV emission seen by HST and JWST (e.g. ALPINE and REBELS surveys). We investigated this using Cosmic Dawn (‘CoDa’) III, a state-of-the-art radiation-hydrodynamics simulation of fully coupled galaxy formation and re-ionisation, which includes a dynamical dust model. Dust-UV offsets are predicted for massive, UV-bright galaxies, up to ∼2 pkpc for the most massive haloes (M_DM > 10^11.5 M_⊙, M_* > 10¹⁰ M_⊙, and M_AB1600 < −21.5). Observed offsets are larger for ALPINE (4.4 < z < 5.9) than REBELS (z > 6.5), consistent with those in CoDa III, where higher-mass haloes appear at lower redshift. These offsets result primarily from severe dust extinction in galactic centres rather than from a misalignment in the dust- and stellar-mass distributions. At the CoDa III spatial resolution (1.65 pkpc at z = 6), dust and stellar components typically remain aligned. We predict that dust emission should therefore align well with stellar rest-frame Near Infra-Red, which is less affected by dust attenuation than the UV. This highlights the importance of dust in shaping galactic appearance. Our simulated dust masses for such bright galaxies agree with observational estimates, but to match the observed UV luminosity function, we reduced our dust attenuation. For SMC-like dust, the bright-end of the UV luminosity function at these redshifts is best fit, for example, with only 7.5% of the dust in CoDa III galaxies. Our massive galaxies thus reproduce the UV spectral slopes of ALPINE and REBELS galaxies, while preserving their dust-UV offsets, but exhibit dust masses lower than those suggested by observations.