THRESHOLDS AND THE CHEMICAL EVOLUTION OF GALACTIC DISKS .2. RADIAL FLOWS

We introduce radial flows into the chemical evolution models of Chamcham, Pitts & Tayler. In this work, the authors used a stability criterion in disc galaxies to define a threshold for star formation and to deduce a self-consistent star formation rate by combining an energy equation for the gas with the standard equations for the chemical evolution of galaxies. These models reproduced the general observable features of spiral galaxies and of the solar neighbourhood, but failed to reproduce the metallicity gradients observed in most galaxies. A biased infall rate increased the gradient, but not sufficiently to correspond with some observations. If the disc is formed out of centrifugal equilibrium, the resulting radial flows will also enhance the composition gradient, as is shown in the present paper.