IGIMF
The evolution of the mass components
The evolution of the mass components
The serrated shape is caused by the simplification that all the stars formed in a 10 Myr star formation epoch have an identical age.
Thus at the beginning of each 10 Myr, the star forms in the simulated galaxy. While about 3 Myr after the formation, the most massive stars start to die and the mass of the alive star drops.