ISM column density vs. redshift for different samples of M_* > 10¹⁰ M_⊙ galaxies as labeled. For ASPECS and COSMOS z ∼ 3.3 galaxies, ISM masses were derived from dust masses, and ISM sizes from UV/optical stellar sizes. For ALPINE galaxies and for z ∼ 6 QSO hosts both ISM masses and sizes were derived from [C II] data. For comparison, we also show for the ALPINE sample the results obtained using ISM masses derived from dust mass and ISM sizes derived from UV/optical stellar sizes (green open pentagons; see text for details). The upper bound of the light blue shaded area shows the median N_H values obtained for the different samples (ASPECS was divided into two redshift bins). The lower bound shows the median N_H values after including a conservative correction for incompleteness and observational biases (see Sect. 5.2). The N_H ∝ (1 + z)⁴ solid curve is obtained by combining empirical trends for the gas mass and galaxy size and is normalized to N_H = 10²¹ cm⁻² at z = 0, which is consistent with the angle-averaged column density of the Milky Way (Willingale et al. 2013) and with the upper limit to the median column density of local massive galaxies as derived from the EDGE-CALIFA survey (Bolatto et al. 2017; big square at z = 0.1). The long-dashed curve running through the light-blue shaded area corresponds to (1 + z)^3.3. The gold shaded area is from Buchner et al. (2017) as derived from the Illustris-1 simulation: lower and upper bounds are for galaxies with M_* > 10¹⁰ M_⊙ and M_* > 10¹¹ M_⊙, respectively. The horizontal dashed line shows the column density corresponding to Compton-thick absorption.