If the ionization of the Ly-
clouds is low enough to permit substantial metal
abundances, then the observations of
Foltz et al. (1984)
demand that they must be
thin sheet like structures. Barcons and Fabian (1987) have discussed
this possibility
and concluded that one might at the same time account naturally for the
distribution
of column densities if such thin
Ly- clouds were flat, rather than
curved, so that
large column densities could arise in clouds observed edge on.
Rees (1980)
noted that
conduction will lead to the rapid evaporation of such clouds unless
magnetic fields
are present. But magnetic fields are desirable in this scenario, since
they would tend
to produce the filamentary or sheet like structures, elongated along the
local field direction.
While it is not necessary that the sheets be fully contiguous,
differences in the
velocities of any component clouds must be limited to
25 km
s-1, both to conform
with the Foltz et al. observations, and the lack of clustering of
Ly- systems in general.
It is interesting to note that a metal line system is also seen in the lensed QSO spectra obtained by Foltz et al. Striking differences are noted between the two lines of sight, implying that two separate clouds are being observed, despite the similarity in redshift. Indeed one of the clouds shows strong lines of low ionization ions which must arise in gas of relatively high density, and consequently the thickness of this cloud must be much less that the separation of the two lines-of-sight.
An even more interesting case is the metal line system at
zabs = 1.1249 in the
spectra of both components of the original lensed QSO 0957+561A, B.
Young et al. (1981)
measured a velocity difference of only 8 ± 11 km
s-1 along the two lines-of-sight
which are separated by 8 kpc. Noting that the observation of absorption lines
of similar strengths and velocities is extremely unlikely if the two
absorbing clouds
belong to a galaxy halo, the authors discuss an origin in a face-on
galaxy. It seems
that this may be an example of a metal line system which has the sheet
like structure suggested for the
Ly- clouds.