Stardust galaxies at z>9: A dust-origin transition behind the excess of UV-bright galaxies

Recent James Webb Space Telescope (JWST) observations suggest that galaxies at z ≳ 9 may be dominated by low-opacity supernova (SN)-produced dust before efficient interstellar-medium (ISM) grain growth becomes established. Such a transition in the dominant origin and opacity of dust could naturally explain the prevalence of galaxies with extremely low dust attenuation as well as the excess of ultraviolet-bright (UV-bright) galaxies relative to most pre-JWST predictions. We investigated whether this transition, combined with a redshift evolution of the star formation efficiency, can account for these observed properties. We developed a physically motivated attenuation framework combining (i) extinction laws for SN dust processed by reverse shocks, (ii) metallicity- and dust-to-metal-dependent opacity scalings, and (iii) porous radiative-transfer geometries allowing partial UV-photon leakage. Unlike outflow-driven scenarios requiring large-scale gas evacuation, our approach preserves gas reservoirs while reducing the effective UV opacity through dust composition and geometry. We used observed and model-motivated galaxy scaling relations to follow how dust mass, metallicity, gas content, galaxy size, and star–dust geometry regulate the far-UV attenuation as galaxies evolve from an early SN-dominated dust regime towards a later ISM grain-growth regime. We find that the observed relation between far-UV attenuation, A_FUV, and stellar mass, M_̊m star, at z>9 is best reproduced by an intrinsic FUV dust opacity of κ_ FUV ^ dust ̊m cm².g^ -1, which is substantially below standard ISM-like values. Within the baseline physical scenario adopted in this work, this low-opacity regime is naturally associated with SN-produced dust processed by reverse shocks and produces very low attenuation even in gas-rich galaxies. It reproduces galaxies with extremely low dust attenuation (GELDAs), which dominate the observed samples at z≳9. Applied to intrinsic UV luminosity functions from semi-empirical and semi-analytic models, our SN-dominated and hybrid attenuation prescriptions suppress mainly the brightest sources, bringing theoretical predictions into agreement with JWST measurements and showing that only modest dust corrections are needed once the intrinsic UV luminosity functions are close to the JWST ones. Our results suggest that the UV-bright galaxy excess at z may reflect a transition in dust origin and opacity during the earliest phases of galaxy evolution. In the most metal-poor systems, the dust content of some GELDAs may trace the chemical 'ashes' of the first stellar generations, potentially including Population III SNe.