Metallicity vs total hydrogen column density. The different lines correspond to the hydrogen column density of a single H₂ cloud illuminated by a UV field for different UV to density ratios I_UV/n as indicated above each line, with I_UV in units of the Draine field and n in cm⁻³. Filled symbols correspond to H₂-bearing systems. Proximate H₂ systems are from this work, proximate DLAs from Ellison et al. (2010), and intervening systems from Noterdaeme et al. (2008) and Balashev et al. (2019).