Effects of dust scattering albedo and 2175-Å bump on ultraviolet colours of normal disc galaxies

We discuss dust properties in the interstellar medium (ISM) of nearby normal galaxies by comparing observations in the ultraviolet (UV) with simulations by a radiative transfer model. The observed UV colours of nearby galaxies show a reddening relative to their expected intrinsic colours. Some authors argued that the Milky Way dust cannot reproduce the reddening because of the prominent 2175-Å absorption bump. Other authors proposed a reduction mechanism of the bump strength in an attenuation law derived from the ratio of the observed intensity to the intrinsic one through an age-selective attenuation (i.e. young stars are more attenuated selectively). We find that the wavelength dependence of the scattering albedo also has a strong effect on the UV colour, an albedo decreasing towards shorter wavelengths (except for the absorption bump range) produces a significant UV reddening. After comparing the observed UV colours of nearby normal galaxies with those expected from radiative transfer simulations assumed several dust models, we find two sorts of dust suitable for these galaxies: (i) dust with a bump and a smaller albedo for a shorter wavelength (except for the bump range) and (ii) dust without any bump but with an almost constant albedo. If very small carbonaceous grains responsible for the common unidentified infrared emission band are also the bump carrier, the former dust is favourable. Finally, we derive mean attenuation laws of various dust models as a function of the UV attenuation, and derive some relations between the UV attenuation and observable/theoretical quantities.