Abstract Strong Mg ii quasar absorption line systems provide us with a useful tool to understand the gas that plays an important role in galaxy formation. In this paper, placing the theories of galaxy formation in a cosmological context, we present semi-analytic models and Monte-Carlo simulations for strong Mg ii absorbers produced in gaseous galactic haloes and/or galaxy discs. We investigate the redshift path density for the Mg ii absorption lines and the properties of galaxy/absorber pairs, in particular the anti-correlation between the equivalent width of Mg ii absorption line and the projected galaxy-to-sightline distance. The simulated result of the mean redshift path density of strong Mg ii systems is consistent with the observational result. The fraction of strong Mg ii systems arising from galaxy disks is predicted to be ~ 10% of the total. There exists an anti-correlation between the absorption line equivalent and the projected distance of sightline to galaxy center and galaxy luminosity. We determined that the mean absorbing radius Rabs ≈ 29 h−1 kpc(LB/LB∗)0.35. After taking selection effects into consideration, this becomes Rabs ≈ 38 h−1 kpc(LB/LB∗)0.18, which is in good agreement with the observational result. This shows the importance of considering selection effects when comparing models with observations.
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