Dust attenuation in disk-dominated galaxies: Evidence for the 2175 Å dust feature

The attenuation of starlight by interstellar dust is investigated in a sample of low-redshift, disk-dominated star-forming galaxies using photometry from GALEX and SDSS. By considering broadband colors as a function of galaxy inclination, we are able to confidently separate trends arising from increasing dust opacity from possible differences in stellar populations, since stellar populations do not correlate with inclination. We are thus able to make firm statements regarding the wavelength-dependent attenuation of starlight for disk-dominated galaxies as a function of gas-phase metallicity and stellar mass. All commonly employed dust attenuation curves (such as the Calzetti curve for starbursts, or a power-law curve) provide poor fits to the ultraviolet colors for moderately and highly inclined galaxies. This conclusion rests on the fact that the average FUV-NUV color varies little from face-on to edge-on galaxies, while other colors such as NUV-u and u - r vary strongly with inclination. After considering a number of model variations, we are led to speculate that the presence of the strong dust extinction feature at 2175 Å seen in the Milky Way extinction curve is responsible for the observed trends. If the 2175 Å feature is responsible, these results would constitute the first detection of the feature in the attenuation curves of galaxies at low redshift. Independent of our interpretation, these results imply that the modeling of dust attenuation in the ultraviolet is significantly more complicated than traditionally assumed. These results also imply a very weak dependence of the FUV-NUV color on total FUV attenuation, and we conclude from this that it is extremely difficult to use only the observed UV spectral slope to infer the total UV dust attenuation, as is commonly done. We propose several simple tests that might finally identify the grain population responsible for the 2175 Å feature.