UV Dust Attenuation in Normal Star-Forming Galaxies. I. Estimating the LTIR/LFUV Ratio

We analyze the dust attenuation properties of a volume-limited, optically selected sample of normal star-forming galaxies in nearby clusters as observed by GALEX. The internal attenuation is estimated using three independent indicators, namely, the ratio of the total infrared to far-ultraviolet emission, the ultraviolet spectral slope β, and the Balmer decrement. We confirm that normal galaxies follow a LTIR/LFUV-β relation offset from the one observed for starburst galaxies. This offset is found to weakly correlate with the birthrate parameter and thus with the galaxy star formation history. We study the correlations of dust attenuation with other global properties, such as the metallicity, dynamical mass, ionized gas attenuation, Hα emission, and mass surface density. Metal-rich, massive galaxies are, as expected, more heavily extinguished in the UV than are small systems. For the same gas metallicity normal galaxies have lower LTIR/LFUV ratios than starbursts, in agreement with the difference observed in the LTIR/LFUV-β relation. Unexpectedly, however, we find that normal star-forming galaxies follow exactly the same relationship between metallicity and ultraviolet spectral slope β determined for starbursts, complicating our understanding of dust properties. This result might indicate a different dust geometry between normal galaxies and starbursts, but it could also be due to aperture effects eventually present in the IUE starbursts data set. The present multiwavelength study allows us to provide some empirical relations from which the total infrared to far-ultraviolet ratio (LTIR/LFUV) can be estimated when far-infrared data are absent.