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We present a synthesis of existing observations of the clustering of QSO absorption systems, including new observations of Mg~II system clustering between closely spaced lines of sight. The clustering of QSO absorption systems can be measured out to redshifts above two, and should provide a key cosmological constraint. Two major results are discussed:
Firstly, the clustering is a strong function of column density; two-point correlation amplitudes for damped systems ($n_H > 10^{20} {\rm cm}^{-2}$) may exceed those for Lyman-$\alpha$ forest clouds ($n_H < 10^{17} {\rm cm}^{-2}$) by as much as four orders of magnitude. This suggests that most absorption systems cannot trace the underlying mass \ -- \ some form of strong ``column-density biasing'' must be taking place at redshifts $\sim 2$.
Secondly, we find that, if the data is representative of conditions everywhere in the universe, high column density absorption systems are far more strongly clustered than are modern galaxies! If, as currently thought, these absorption systems are associated with the progenitors of modern galaxies, this will pose a great problem for hierarchical clustering models of large-scale structure formation such as CDM. If, alternatively, high column-density absorption systems are not associated with galaxies, a new population of highly clustered high redshift objects is required.
