Fig. 4
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Time evolution of the spiral arms density contrast. We considered the radial ranges 4–8 kpc (top) and 8–12 kpc (bottom). The points show the relative ratio between the mean mass surface density of the arms and the disc average mass surface density of gas (circles) and of all stellar populations together (stars) as a function of time. Colour changes with time as in Figs. 5, 6, and 11.
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