The evolution of stellar exponential discs

Models of disc galaxies which invoke viscosity-driven radial flows have lon g been known to provide a natural explanation for the origin of stellar exp onential discs, under the assumption that the star formation and viscous ti me-scales are comparable. We present models which invoke simultaneous star formation, viscous redistribution of gas and cosmologically-motivated gaseo us infall and explore the predictions such models make for the scale-length evolution and radial star formation history of galactic stellar discs. Whi le the inclusion of viscous flows is essential for ensuring that the stella r disc is always exponential over a significant range in radius, we find th at such flows play only a minor role in determining the evolution of the di sc scalelength. In models in which the main infall phase precedes the onset of star formation and viscous evolution, we find the exponential scaleleng th to be rather invariant with time, with the bulk of the disc stars at all radii out to similar to5 scalelengths being relatively old (i.e. ages grea ter than or similar to6-8 Gyr for an assumed disc age of 11 Gyr). On the ot her hand, models in which star formation/viscous evolution and infall occur concurrently result in a smoothly increasing scalelength with time, reflec ting the mean angular momentum of material which has fallen in at any given epoch. The disc stellar populations in these models are predominantly youn g (i.e. ages less than or similar to5 Gyr) beyond a few scalelengths. In bo th cases, viscous flows are entirely responsible for transporting material to very large radii. Our results are robust for a range of currently popula r star formation laws and infall prescriptions. We discuss existing observa tional constraints on these models from studies of both local and moderate redshift disc galaxies. In particular, a good agreement is found between th e solar neighbourhood star formation history predicted by our infall model and the recent observational determination of this quantity by Rocha-Pinto et al.