Properties of the sharp metal-rich absorption lines observed in QSO spectra
Many reviews have been devoted to the topic of the absorption lines in QSO (see e.g., Weymann et al., 1981), revealing the importance of these absorption line systems as a tool to probe the environment of QSO and galaxies, to study any intervening material not otherwise detectable and to investigate the cosmological evolution of these populations. We only consider here the sharp metal-rich absorption systems, now fairly generally believed to be of intervening origin. We outline their properties, with emphasis on their degree of ionization, and compare them to those of halos or extended disks around nearby galaxies. We present some evidence for cosmological evolution of the density per unit redshift z of these systems and try to find whether the differences between the high z absorbers and galaxy halos are a consequence of cosmological evolution in the ionization state or whether there are two populations or two phases among the metal-rich systems. In the assumption of ionization of the absorbers by the UV diffuse background due to QSO emission, we present results obtained from the consideration of models of photoionized clouds and outline the constraints on the ionization parameter when comparing the models to the observed absorption line equivalent widths and the derived column densities. Finally, we briefly discuss the possibility of a link between the metal-rich systems and the Lyα forest.
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