Towards an understanding of the physical nature of MgII absorption systems

We discuss issues concerning the physical nature of intervening MgII quasar absorption systems in light of results from our recent surveys using SDSS edr QSO spectra and data obtained at the mmt. These surveys indicate an excess of weak (w₀^λ2796 ≲ 0.3å) systems relative to the exponential ∂ n/∂ w₀ distribution of stronger systems. The incidence of intermediate-strength lines shows remarkably little evolution with redshift, thereby constraining models for the nature of the clouds comprising these absorbers. The total distribution does evolve, with the incidence decreasing with decreasing redshift in a w₀^λ2796-dependent rate (the strongest systems evolve the fastest). This suggests that multiple populations that evolve at different rates contribute to the incidence in a w₀^λ2796-dependent manner. We also present two images of fields containing unprecedented "ultra-strong" (w₀^Λ2796 ≥ 4.0å) MgII absorbers.