There exist impressively large differences in the clustering properties of the metal line and the Ly- systems. It was shown by Sargent et al. (1980) that the metal line systems cluster strongly on velocity scales of a few hundred km s-1, while the Ly- systems cluster much less strongly, if at all. Sargent et al. (1980) suggested that the metal line systems arise in galaxies and thus cluster like galaxies, while the Ly- systems are a separate population with a widespread intergalactic distribution.
Considering the absorption system clustering from a comparative perspective in the context of cold dark matter dominated models, Salmon and Hogan (1986) arrive at two possible interpretations. If = 0.2, the metal systems can be associated with galaxies which are an unbiased sample of the mass distribution. Some unknown mechanism must stop the Ly- systems from following the gravitational potential of the galaxies.
In an = 1 model the correlation function is very weak at the redshifts of the absorbers. The Ly- clouds could be an unbiased sample of the mass, but the metal line systems would have to be more strongly clustered than the mass, either because they are biased like galaxies, or because non-gravitationally induced velocities are causing the clustering. A particularly interesting result of this study was that the velocity correlation was found to be only weakly, if at all, dependent on the number density, diameter or mass of the absorption system cloud, implying that gravitational clustering alone can not account for the differences between the Ly- and the metal line systems.
This is consistent with the suggestion of York et al. (1986) that much of the clustering of the metal line systems might be ascribed to the dynamics of explosions in dwarf galaxies which are actively forming stars.
It will be possible to test these ideas by evaluating the variation of the absorption system clustering with epoch, since the growth of clustering of the Ly- systems will be particularly dramatic in an = 1 universe, while hydrodynamic effects arising during galaxy formation should die out.