The Physical Origins of Scatter in the Dust Attenuation Scaling Relation of Star-Forming Galaxies: Star–Dust Geometry

Abstract Characterising the physical drivers of dust attenuation in star-forming galaxies (SFGs) is essential for interpreting their spectral energy distributions and star formation histories. The infrared excess (IRX≡LIR/LUV) follows a universal scaling relation with metallicity, star formation rate, galaxy size, and inclination. However, the origin of the scatter around this relation remains poorly understood. We revisit this relation using ∼32,000 local SFGs from SDSS, GALEX, and WISE, and investigate why some galaxies deviate systematically from the best-fit relation. We find that the deviations are systematically linked to UV luminosity. UV-faint SFGs (log (LUV/L⊙) ≤ 9) exhibit a median IRX excess of +0.23 dex, while UV-bright SFGs (log (LUV/L⊙) ≥ 10) show a median deficit of −0.20 dex relative to the relation defined by the dominant UV-intermediate population (86.38 per cent of the sample). These offsets are not driven by metallicity, total infrared luminosity, or specific star formation rate. Instead, the deviations are closely linked to systematic differences in star–dust geometry. UV-faint SFGs are compact (median Re = 2.90 kpc) and preferentially viewed edge-on (median b/a = 0.41), leading to high effective dust column densities along the line of sight. UV-bright SFGs are extended (median Re = 5.99 kpc) and preferentially viewed face-on (median b/a = 0.74), allowing UV photons to escape efficiently. Galaxies with significant bulge components (B/T > 0.4) populate the envelope of the relation, exhibiting larger scatter. We conclude that the scatter in the IRX relation can be largely explained by variations in the three-dimensional star–dust geometry, with UV luminosity acting as an effective tracer of these geometric and physical differences.