Hydrogen column densities as a function of density. The model assumes local hydrostatic and thermal equilibrium, metallicity ﹩Z=0.1\,Z_{\odot }﹩, dust‐to‐metals ratio 0.5 times the Galactic value, and the model for the ﹩z=3﹩ UV/X‐ray background radiation of Haardt & Madau (2001). There is a gap around ﹩n_{\mathrm{H}\,}\sim 10^{-1}\,\mathrm{cm}\,^{-2}﹩ because there are no stable solutions in this region.