Density contrast of spiral arms for different populations and times. Two radial ranges are considered: 4–8 kpc (top) and 8–12 kpc (bottom). The lines show the relative ratio between the mean mass surface density of the arms and the disc average mass surface density as a function of stellar age. Each line corresponds to a different time in the simulation (with darker colours meaning earlier times, as in Fig. 4). Crosses linked with dotted lines to guide the eye correspond to the gas density contrast at the corresponding time.