ON THE EVOLUTION OF DAMPED Lyα SYSTEMS TO GALACTIC DISKS

The mean metallicity of the thick disk of the Galaxy is 0.5 dex higher than that of the damped Lyα systems. This has been interpreted to argue that stars in the former do not arise out of gas in the latter. Using new metallicity and H I column-density data we show the metal-rich damped systems do contain sufficient baryons at the thick-disk metallicity to account for the stellar masses of thick disks. Comparing our kinematic data with the metallicities we show that damped Lyα systems exhibiting the largest profile velocity widths, ∆v, span a narrow range of high metallicities, while systems with small ∆v span a wider range of metallicities. This is naturally explained by passage of the damped Lyα sightlines through rapidly rotating disks with negative radial gradients in metallicity. The systematically lower N(H I) of systems with high ∆v indicates (a) the gaseous disks have centrally located holes, and (b) an apparent inconsistency with the protogalactic clump model for damped Lyα systems. The higher metallicity of systems with low N(H I) further implies that stars rather than gas dominate the baryonic content of the most metal-rich damped systems.