Density required to produce H₂ as a function of the distance to the quasar. We assumed here a typical situation, with column density equating the median observed value (log N(H I) = 21.3), assuming (red) and (purple) and a quasar with the median luminosity observed at the Lyman-Werner wavelength range. The different curves are when including a local UV field, in units of Draine field (χ_loc = 0, 1, 10).