The exact nature of the Ly- clouds which are responsible for the Ly- forest discovered by Lynds (1971) is still controversial. It is clear that they are cosmologically distributed, intervening objects which are poor in heavy-elements. Given the present uncertainties in their properties, the clouds could be dwarf irregular galaxies (Fransson and Epstein 1982), primordial intergalactic clouds (Arons 1972; Peterson 1978; Sargent et al. 1980; Black 1981), intergalactic shocks (Chernomordnic and Ozernoy 1983) or even the extended primordial halos of normal galaxies (Bahcall and Spitzer 1969). More recently, Rees (1986) has proposed that the clouds are confined by 'mini-halos' of the cold dark matter whose existence has been invoked to reconcile the lack of observed anisotropy in the microwave background radiation with the existence of galaxies, clusters and superclusters today (see also Rees 1988). Hogan (1987) has taken Tytler's arguments in favor of small, dense clouds seriously and has suggested that the Ly- forest results from shocks generated by collisions between thermally unstable proto-galactic clouds. On the other hand, Bond, Szalay and Silk (1987) see the clouds as pressure-driven relics of primordial density fluctuations. On this scenario, while more massive clouds are confined by the cold dark matter, the Ly- clouds are unstable entities whose expansion has been triggered by the onset of QSO and galaxy formation at a presumed redshift of around z 4.
If the Ly- forest lines are indeed intergalactic in origin, there are three possibilities regarding their stability:
As Ostriker (1988) elaborates, freely expanding clouds are ruled out because their expansion velocities are too big for the observed line widths while for self-gravitating clouds the collapse times are too short.
This leaves pressure-confined clouds as proposed by Sargent et al. (1980) and Ostriker and Ikeuchi (1983). These latter authors devised a model in which the clouds originate from a shock-heated intergalactic medium containing fragmentary shells. On this hypothesis, the most massive condensations form galaxies while the lowest mass ones evaporate. This leaves a narrow range of 107 to 108 M in equilibrium at z = 2.5. The clouds evaporate and dissolve as the Universe expands and the pressure of the intergalactic medium goes down.