Hydrogen number density of host cells at the time of Pop II star formation (n_H, SF). Different runs are represented by different color codes, as indicated in the legend. The average density is indicated by an horizontal dashed line. The gray dashed curve corresponds to the minimum density required to create a star particle of mass 500 M_⊙ in the GTT-Fid without violating the numerical condition that no more than 90% of a cell gas mass is converted into stars per fine time step.