(a–d) Amplitude of fluctuations at ﹩\sigma _{\mathrm{DLA}\,}\,\left(z=4\right)﹩ within a Gaussian window of mass corresponding to halos of 50 km s−1 Mpc−1 circular velocity. Cosmologies are as per Fig. 3. The shaded region indicates cosmologies where the neutral gas in halos of ﹩v_{c}> 50\,\mathrm{km\,}\,\mathrm{s}\,^{-1}\mathrm{\,Mpc}\,^{-1}﹩ (using the prescription of Klypin et al. 1995 and ﹩f_{\mathrm{H}\,\,{}^{{\rm{\small I}}}}=1﹩) exceeds that observed in damped Lyα systems. The region to the left of the dashed line is the allowed region for ﹩f_{\mathrm{H}\,\,{}^{{\rm{\small I}}}}=0.1﹩. (e–h) Amplitude of fluctuations at ﹩\sigma _{\mathrm{Ly}\,\alpha }\,\left(z=3\right)﹩ for a Gaussian window of radius ﹩0.0416( \Omega _{0}h^{2}) ^{-1/ 2}\mathrm{\,Mpc}\,﹩, suggested by Gnedin (1997) as a indicator of the slope of the column‐density distribution of the Lyα forest. The region ﹩\sigma _{\mathrm{Ly}\,\alpha }> 1.5﹩ is shaded. Cosmologies are as in Fig. 3.