2001ApJ...563..867S

Recently, it has been suggested, based on two-dimensional simulations, that giant holes in the intercloud medium may be a consequence of the nonlinear evolution of combined gravitational and thermal instabilities alone, without any stellar energy feedback. Here we investigate the vertical structure of the gas in the disk. It is found that in the strict two-dimensional mapping, the H I disk in starless models would be extremely thin, a few parsecs, because of the excessive radiative cooling. Thus, additional physical processes should be invoked in order to have both significant density substructure, produced by large-scale instabilities, and admissible scale heights. Different possibilities are explored, but none are found satisfactory, indicating that models based mainly on gravitational plus thermal instabilities present severe difficulties in explaining the observed giant holes in shell-filled galaxies.