On the cause of metallicity gradients in spiral galaxies

We investigate particular models for the chemical evolution of galactic discs in an attempt to account for the observed behaviour of metallicity with surface density, and hence explain abundance gradients. The models include simple Schmidt star formation rate laws, models in which the formation of massive stars is suppressed below a threshold interstellar gas density, and models with self-regulation of star formation. Except for the ones with self-regulating star formation, these models are not particularly successful unless rather extreme parameter choices are made. The failure is, perhaps surprisingly, worse for gravitational instability thresholds than for some more arbitrary density thresholds. We suggest that the modulation of star formation rate by additional terms dependent, explicitly or implicitly, on radius may be required to generate the observed gradients.