Reexamination of the Infrared Excess-Ultraviolet Slope Relation of Local Galaxies

Abstract

The relation between the ratio of infrared (IR) and ultraviolet (UV) flux densities (the infrared excess: IRX) and the slope of the UV spectrum (β) of galaxies plays a fundamental role in the evaluation of the dust attenuation of star-forming galaxies, especially at high redshifts. Many authors, however, have pointed out that there is a significant dispersion and/or deviation from the originally proposed IRX-β relation depending on sample selection. We reexamined the IRX-β relation by measuring the far- and near-UV flux densities of the original sample galaxies with GALEX and AKARI imaging data and constructed a revised formula. We found that the newly obtained IRX values were lower than the original relation because of the significant underestimation of the UV flux densities of the galaxies, caused by the small aperture of IUE. Furthermore, since the original relation was based on IRAS data that covered a wavelength range of λ = 42-122 μm, we obtained an appropriate IRX-β relation with total dust emission (TIR): log (L _TIR/L _FUV) = log [10^{0.4(3.06 + 1.58β)} - 1] + 0.22 using the data from AKARI, which has wider wavelength coverage toward longer wavelengths. This new relation is consistent with most of the preceding results for samples selected at optical and UV, though there is significant scatter around it. We also found that even the quiescent class of IR galaxies follows this new relation, though luminous and ultraluminous IR galaxies distribute completely differently as previously thought.