Metal abundances have been estimated for less than a dozen absorption line systems. The main impediment is the scarcity of systems which show enough lines from enough ions to allow simultaneous estimates of both ionization and abundances. Difficulties are also encountered with inadequate spectral resolution, line blending, the existence of multiple subcomponents, and slight differences in the velocity distributions of the different ions in a given system.
Published abundance estimates range from near solar, to about 0.01 of solar abundances. These estimates are sufficiently accurate to show that abundances do differ, but they do not allow a determination of the full range or distribution. Moreover they are unlikely to be fully representative of all metal line systems because estimates are usually made for only those systems which show the largest number of metal lines, a bias which will lead to above average abundances.
Metal line systems are sufficiently common that only a few percent can occur
within the Holmberg radii of galaxies. We should expect that abundances
will usually
be at levels anticipated for the outer regions of galactic haloes at
generally high
redshifts. Values around 0.01 solar might be appropriate, depending on
the history of
enrichment. An evaluation of the distribution of abundances as a
function of epoch
can be expected in the coming decade. These results will be of extreme
importance
in the determination of the sequence of metal enrichment during and
prior to galaxy
formation. For example, if it is found that metal systems with high
abundances are common at the largest redshifts (z
4), then enrichment by
Population III stars
would be indicated for theories which have galaxies collapsing at later epochs.
Attempts to set upper limits on the abundances of the
Ly-
systems are thwarted
by our lack of precise knowledge of their level of ionization, and hence
their total column densities. Two approaches have been followed.
Ly- systems typically have
neutral hydrogen column densities of N(HI) = 1014 cm-2
(Sargent et al. 1980;
Carswell et al. 1987).
Even if these systems have very high
ionizations corresponding to total column densities of logN = 19, it is
observationally
difficult to set abundance limits which are substantially below those
found in the
metal line systems. Norris, Hartwick and Peterson (1983) attempted to overcome
this problem by summing portions of the spectra of two QSOs, shifted to
align the
expected positions of metal lines at the redshifts of some 65
Ly- systems. They did
not detect CIV or NV, but OVI was marginally present. Searches in higher
quality spectra of other QSOs by
Sargent and
Boksenberg (1983) and
Norris and Peterson
(1986)
have failed to confirm the OVI detection, leading to typical
upper limits on the
abundances of metals of about 0.03 solar, a limit which is sensitive to
the assumed level of ionization.
Impressive upper limits on the metal abundances have also been determined for
two Ly- systems which have
logN(HI) = 17, amongst the largest N(HI) known for
Ly- systems. In both cases the
limits are about 0.001 solar for an assumed level of ionization
(Sargent and
Boksenberg 1983;
Chaffee et al. 1986).
The basic assumption
is that these rare systems with logN(HI) = 17 should have about the same
level of
ionization as the typical systems with logN(HI) = 14. The large N(HI)
then corresponds
to large total column densities needed to justify the low
abundances. Chaffee et al.
quantify this argument by assuming that the
Ly- systems are in
pressure equilibrium
with a hot low density intergalactic medium. All clouds at a given
redshift should
then have about the same density and ionization. It follows that the
logN(HI) = 17
systems will be about 1000 times the size of the typical
Ly- systems, even though
they are only about 1% as common. Since detection probability is
proportional to
cross-sectional area, the logN(HI) = 17 systems must then represent only
1 in 108 of
Ly- absorbing clouds, and they
would have 109 times the mass of a typical cloud. It
is then far from obvious that these should be considered typical clouds.
Nevertheless these results are extremely important because they show that some
systems probably do have low abundances. This follows from the observation that
some Ly- and some metal line
systems do have sizes in excess of 1 kpc, and it is
reasonable to assume that the same applies to those systems with logN(HI) = 17,
which are intermediate in N(HI).
In summary, there is no direct observational evidence showing that the typical
Ly- system has a metal
abundance which is well below the range shown by metal
line systems, but there is at least a 3 order of magnitude range in
metal abundances for the narrow line systems.