  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. 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 2175A seen in the Milky Way (MW) extinction curve is
responsible for the observed trends. 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 2175A feature.
