Temperature evolution for different metallicities Z and with the cooling contribution by ionized or neutral metals alternatively switched off. The UV flux is switched on. Top panel: no cooling by ionized metals. Bottom panel: no cooling by neutral metals. In both panels the meaning of the lines is as follows: the dot-dashed line is for Z = 1, the dashed line for Z = 10^-1, the dotted line for Z = 10^-2, and the solid line for Z = 10^-10. See the text for more details.