Chemo-spectrophotometric evolution of spiral galaxies -- III. Abundance and colour gradients in discs

We study the relations between luminosity and chemical-abundance profiles of spiral galaxies, using detailed models for the chemical and spectrophotometric evolution of galactic discs. The models are `calibrated' on the Milky Way disc and are successfully extended to other discs with the help of simple `scaling' relations, obtained in the framework of semi-analytic models of galaxy formation. We find that our models exhibit oxygen abundance gradients that increase in absolute value with decreasing disc luminosity (when expressed in dex kpc 21 ) and are independent of disc luminosity (when expressed in dex scalelength 21 ), both in agreement with observations. We notice an important strong correlation between abundance gradient and disc scalelength. These results support the idea of `homologous evolution' of galactic discs.