The evolution of stellar exponential discs

Abstract

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 time-scales 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 scalelength 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 only a minor role in determining the evolution of the disc scalelength. In models in which the main infall phase precedes the onset of star formation and viscous evolution, we find the exponential scalelength to be rather invariant with time, with the bulk of the disc stars at all radii out to ∼5 scalelengths being relatively old (i.e. ages ≳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 scalelength 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 (i.e. ages ≲5 Gyr) beyond a few scalelengths. 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.