Annu. Rev. Astron. Astrophys. 1998. 36: 267-316 Copyright © 1998 by Annual Reviews. All rights reserved

4.2. Gravitational Confinement: Selfgravity

Self-gravitating baryonic clouds were suggested by Melott (1980) as an alternative to the pressure confinement model. Black (1981) investigated in detail the physical structure of such objects. clouds. He found that (quasi-)stable clouds with properties consistent with the observations have to be extended (~ 1 Mpc), and must either be truncated by an external medium, or be large enough to overlap, providing their own boundary pressure. In this model the appearance of the intergalactic medium as a forest of lines is more due to the strong internal gradients of the neutral gas density, than to a sharp transition between separate entities. The density of the intergalactic medium along the LOS undulates, and there is no real difference between an intercloud medium and the clouds. The huge sizes would also have been able to reconcile a larger mass density of the intergalactic medium with the observed cloud parameters, whereas pressure confined clouds would contain only a small fraction of all baryons. The model met with scepticism because the large sizes appeared to contradict the scant observational evidence. However it may also have other problems, such as reproducing the column density distribution (Petitjean et al 1993a).