Chemo-spectrophotometric evolution of spiral galaxies - II. Main properties of present-day disc galaxies

We study the chemical and spectrophotometric evolution of galactic discs wi th detailed models calibrated on the Milky Way and using simple scaling rel ations, based on currently popular semi-analytic models of galaxy formation . We compare our results with a large body of observational data on present -day galactic discs, including disc sizes and central surface brightness, T ully-Fisher relations in various wavelength bands, colour-colour and colour -magnitude relations, gas fractions versus magnitudes and colours and abund ances versus local and integrated properties, as well as spectra for differ ent galactic rotational velocities. Despite the extremely simple nature of our models, we find satisfactory agreement with all those observables, prov ided that the time-scale for star formation in low-mass discs is longer tha n for more massive ones. This assumption is apparently in contradiction wit h the standard picture of hierarchical cosmology. We find, however, that it is extremely successful in reproducing major features of present-day discs , like the change in the slope of the Tully-Fisher relation with wavelength , the fact that more massive galaxies are on average 'redder' than low-mass ones (a generic problem of standard hierarchical models) and the metallici ty-luminosity relation for spirals. It is concluded that, on a purely empir ical basis, this new picture is at least as successful as the standard one. Observations at high redshifts could help to distinguish between the two p ossibilities.