THRESHOLDS AND THE CHEMICAL EVOLUTION OF GALACTIC DISKS

Observations by Kennicutt suggest that significant star formation in g alactic discs only occurs when a criterion for disc instability is sat isfied and that, when star formation is occurring, departure from the marginal state is not great. We have studied galactic disc models in w hich there is no star formation until the disc has a threshold surface density at which an instability criterion is satisfied. We assume tha t the rate of the subsequent star formation is just such as to keep th e criterion marginally satisfied. Our models contain many approximatio ns and free parameters, but they do have properties which resemble tho se of real galactic discs. These include a slow variation of the star formation rate with time and a slow change in both space and time of t he gas random velocity. There is good general agreement with the obser ved stellar age/metallicity relation. After a time, at any radial posi tion, the rate of star formation is close to a power law in the gas su rface density. Because of the threshold, no stars are produced until s ome time after the disc starts forming and, if the model were valid, t here would therefore be no luminous discs at very high redshift. Moreo ver, there should not be a large density of hidden matter in the form of elementary particles in the solar neighbourhood. The threshold mode l cannot readily explain the G-dwarf distribution in the solar neighbo urhood, and it is necessary to suppose that the disc starts with a non -zero initial metallicity. The model in which gas infall proceeds on t he same time-scale at all radii does not produce abundance gradients a s large as those observed. There is some improvement if the infall is biased, occurring more slowly at larger radii.