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If the ionization of the Ly-alpha 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-alpha 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 leq 25 km s-1, both to conform with the Foltz et al. observations, and the lack of clustering of Ly-alpha 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-alpha clouds.

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