Ultraviolet dust attenuation in spiral galaxies: the role of age-dependent extinction and the initial mass function

We analyse the attenuation properties of a sample of ultraviolet (UV) selected galaxies, with the use of the spectrophotometric model GRASIL. In particular, we focus on the relation between dust attenuation and the reddening in the UV spectral region. We show that a realistic modelling of geometrical distribution of dust and the different population of stars can explain the UV reddening of normal spiral galaxies also with a standard Milky Way dust. Our results clearly underline that it is fundamental to take into account that younger stars suffer a higher attenuation than older stars (the age-dependent extinction) because stars are born in more-than-average dusty environments. In this work, we also find that the concentration of young stars on the galactic plane of spirals has a relevant impact on the expected UV colours, impact that has not been explored before this paper. Finally, we discuss the role of the initial mass function (IMF) in shaping the relation between UV reddening and dust attenuation, and we show that a Kroupa IMF is more consistent with observed data than the classical Salpeter IMF.