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.