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abstract
Of the various types of heavy element absorption lines that appear in quasar spectra, considerable controversy has surrounded the interpretation of the so-called {$z_{\rm abs} \approx z_{\rm em}$} systems, defined as C{\small IV} doublets at redshifts within $\Delta v=\pm5000$~km~s$^{-1}$\ of the emission redshift. As reviewed most recently by Foltz, Chaffee, Weymann, and Anderson, 1988, in QSO Absorption Lines: Probing the Universe Cambridge University Press, Cambridge, p.53, it is not presently clear whether this class of absorption system is external or intrinsic in nature. One possibility is that such systems arise in the halos of galaxies located in a cluster that includes the host galaxy of the quasar (Weymann, Williams, Peterson, and Turnshek, 1979, ApJ, 234 , 33). Alternatively, such systems could be related to the so-called broad absorption-line systems, and caused by absorption in matter ejected by the quasar itself (Turnshek, 1988, in QSO Absorption Lines: Probing the Universe Cambridge University Press, Cambridge, p.17.
Recent studies of {$z_{\rm abs} \approx z_{\rm em}$} systems (Savaglio, D'Odorico, and M{\o}ller, 1994, A\&A, 281 , 331; Petitjean, Rauch, and Carswell, 1994, A\&A, 291 , 29; M{\o}ller, Jakobsen, and Perryman, 1994, A\&A, 287 , 719) have revealed that the {$z_{\rm abs} \approx z_{\rm em}$} systems are in fact distinctly different from corresponding intervening systems, in that they generally have extremely high metallicities. This suggests that those systems are more likely to be related to BAL systems, rather than to intervening absorbers. I shall briefly review the evidence for the metallicities of those systems, as well as the inverse luminosity correlation (M{\o}ller, Jakobsen, 1987, ApJ-Lett 320 , L75). Both of these results may provide important clues to the distribution, nature, and history of the absorbing gas in the AGN environment.
