Models of disc galaxies which invoke viscosity-driven radial flows have long been known to provide a natural explanation for the origin of stellar exponential discs , under the assumption that the star formation and viscous timescales are comparable .
We present models which invoke simultaneous star formation , viscous redistribution of gas and cosmologically-motivated gaseous infall and explore the predictions such models make for the scale length evolution and radial star formation history of galactic stellar discs .
While the inclusion of viscous flows is essential for ensuring that the stellar disc is always exponential over a significant range in radius , we find that such flows play essentially no role in determining the evolution of the disc scale length .
In models in which the main infall phase precedes the onset of star formation and viscous evolution , we find the exponential scale length to be rather invariant with time , with the bulk of the disc stars at all radii out to \sim 5 scale lengths being relatively old ( ie .
ages \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 6 - 8 Gyr for an assumed disc age of 11 Gyr ) .
On the other hand , models in which star formation/viscous evolution and infall occur concurrently result in a smoothly increasing scale length with time , reflecting the mean angular momentum of material which has fallen in at any given epoch .
The disc stellar populations in these models are predominantly young ( ie .
ages \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 5 Gyr ) beyond a few scale lengths .
In both cases , viscous flows are entirely responsible for transporting material to very large radii .
Our results are robust for a range of currently popular star formation laws and infall prescriptions .
We discuss existing observational constraints on these models from studies of both local and moderate redshift disc galaxies .
In particular , a good agreement is found between the solar neighbourhood star formation history predicted by our infall model and the recent observational determination of this quantity by Rocha-Pinto et al ( 2000 ) .