Maps of a halo at z = 18 of mass M_vir = 3 × 10⁶ M_⊙ and T_vir = 5.7 × 10³ K. The first map shows the gas density, the middle one the gas temperature and the rightmost one a map of H₂ mass fraction. Every property is computed from a mass-average in a slice 30 pc deep. The red circle is the virial radius of the halo. The density map shows the gas collapsing from larger scale, channeling along filaments into the halo. At roughly the virial radius, the gas is heated and starts to pile up. Then, as molecule formation is triggered, gas starts to cool down. Despite the much higher density in the centre of the halo of ∼10⁵ cm⁻³, the gas is much colder, reaching a few hundred K. Here, in the densest region, the H₂ mass fraction reaches ∼10⁻³. The gas temperature correlates strongly with the H₂ fraction, revealing the importance of H₂ for cooling.