Competition between dust scattering albedo and 2175 A bump for ultraviolet colours of nearby disc galaxies

Observed ultraviolet (UV) colours of nearby disc galaxies show a reddening relative to their expected intrinsic colours. Since the 2175 \AA bump found in the Milky Way's dust extinction law blues the UV colours, it might suggest that dust in nearby disc galaxies does not have the bump and that the Milky Way is exceptional. However, this conclusion can be modified by the effect of scatterings. If the scattering albedo decreases towards shorter wavelengths, observable UV colours redden. An extensive comparison between observed UV colours and those expected from radiative transfer simulations shows two types of dust suitable for nearby disc galaxies: (1) dust with a bump and a smaller albedo for a shorter wavelength (except for the bump range), and (2) 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 favorable.