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

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, pointed out that there is a significant dispersion and/or deviation from the originally proposed IRX-βrelation depending on sample selection. We reexamined the IRX-βrelation 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, Further, since the original relation was based on IRAS data which covered a wavelength range of λ= 42--122\mum, using the data from AKARI which has wider wavelength coverage toward longer wavelengths, we obtained an appropriate IRX-βrelation with total dust emission (TIR): \log(L_{\rm TIR}/L_{\rm FUV}) = \log [10^{0.4(3.06+1.58β)}-1] +0.22. This new relation is consistent with most of the preceding results for samples selected at optical and UV, though there is a 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 well known before.