Best fit to the cosmological H  i mass density of Crighton et al. (2017) [Ω_{H i} = 4.0 × 10^-4(z_abs + 1)^0.60 – dotted line, top panel] and the mean column density obtained from this (bottom panel). The abscissa is mapped to log ₁₀(z_abs + 1), in order to demonstrate the contrast between the evolution of N_{H i} and the star formation density (Hopkins & Beacom 2006) [solid curve], where ψ_∗ is arbitrarily shifted on the ordinate but retains the relative scaling (right hand scale).