5.3. Damped Ly Systems
The N(HI) > 1020.3 cm-2 systems are of particular interest because it is possible to observe many different chemical elements (such as Zn, Cr, Fe, Mn, and Ni) in these objects back to high redshift. Metallicities and abundance patterns can be studied and compared to those of old stellar populations in the Milky Way. Back to z = 3, the metallicity in DLAs, as measured by the undepleted element Zinc, is about 10% of the solar value, but it may decline at z > 3. The identity of sites responsible for DLAs at high z remains controversial, but they do contain most of the neutral Hydrogen in the universe, from which most of its stars form. The kinematic structure of the absorption profiles of neutral and low ionization species is consistent with the rotation of a thick disk, so that it is possible that these are the z = 3 progenitors of normal spiral galaxies. However, this signature is not unique. It could also be the consequence of directed infall in an hierarchical structure formation scenario. The higher ionization species show complex kinematics which vary in relation to those of the lower ionization gas; in some systems they appear to trace relatively similar structure, and in others there are clearly several different phases.
At low redshift, many of the galaxies that are responsible for the DLA absorption can be directly identified. These galaxies are a heterogeneous population. They are not just the most luminous galaxies, but include dwarf and low surface brightness galaxies, and even cases where no galaxy has been identified to sensitive limits. Damped Ly absorption does not trace the most luminous objects, but rather it traces the largest neutral gas reservoirs. An additional selection effect may be important. The most dust-rich galaxies that have the potential to produce DLA absorption could produce enough extinction that their background quasars will not be included in quasar surveys. In this way, the population of DLAs that are actually observed could be significantly biased against dusty galaxy hosts.