Diversity of Galaxy Dust Attenuation Curves Drives the Scatter in the IR X-β Relation

Abstract

We study the drivers of the scatter in the IR excess (IRX)–β relation using 23,000 low-redshift galaxies from the GALEX–SDSS–WISE Legacy Catalog 2 (GSWLC-2). For each galaxy, we derive, using CIGALE and the spectral energy distribution+LIR fitting technique, the slope of the dust attenuation curve and the strength of the UV bump, plus many other galaxy parameters. We find that the IRX–β scatter is driven entirely by a wide range of attenuation curves—primarily by their slopes. Once the slope and the UV bump are fixed, the scatter in the IRX–β relation vanishes. The question of the IRX–β scatter is a direct manifestation of the more fundamental question of the diversity of dust attenuation curves. The predominant role of the attenuation curve is the consequence of a narrow range of intrinsic UV slopes of star-forming (SF) galaxies. Galaxies with different specific star formation rates (sSFRs) or population ages do not show strong trends in the IRX–β diagram because their attenuation curves are, on average, similar. Similarly, there is no shift in the IRX–β locus between starbursts and normal SF galaxies, both types having, on average, steep attenuation curves. Optical opacity is identified as the strongest determinant of the attenuation curve slope and consequently of the IRX–β diversity. Despite the scatter, the use of an average IRX–β relation is justified to correct SFRs, adding a random error of $\lesssim$ 0.15 dex. The form of the local correspondence between IRX–β and attenuation curves is maintained at high redshifts as long as the evolution of the intrinsic UV slopes stays within a few tenths.