The three categories of star formation histories observed: sustained, extended and quenched. The colours code the model galaxy velocity dispersion. The dotted black curves show the redshift dependent hydrogen photo-heating rate due to the UV-background photons. The curves are normalized so that the peak of the photo-heating rate coincides with the top of the y-axis. To illustrate the change in intensity and length of the star formation histories between the three categories, the shaded rectangles in panels a and b have dimensions corresponding to the maximal x- and y-axis extensions of the panels b and c, respectively.