Quasar absorption-line systems and the chemical evolution of galactic discs

Absorption-line clouds sit in an external field of ionizing radiation. As the intensity of the radiation declines, ionized gas becomes neutral and star formation can occur in it. Provided that heavy elements produced in the stars can be largely excluded from the ionized gas, this provides a natural infall model of chemical evolution. Whether or not such a separation is possible probably depends on the presence of a magnetic field in the clouds. An idealized model of disc evolution is calculated. The results suggest that certain features of the model can give rise to a relationship between star formation and metallicity which avoids the G-dwarf problem.