Various characteristic time-scales as a function of gas density in the 10⁻⁴ (top), 10⁻³ (middle) and 10⁻² Z_⊙ run (bottom). Shown is the free-fall time (solid black line), the sound crossing time of the pressure scaleheight (red dashed line – see the text), the compression time (green triple–dot–dashed line – see the text) and the cooling time t_cool = 3nk_BT/2Λ (blue solid line enclosed by a thicker line). The colour of the thicker line enclosing the cooling time indicates the identity of the most effective coolant. Light blue refers to metal line cooling, light red is the cooling by H₂ and HD, and green is Lyα cooling.