Distribution of star formation rate surface density as a function of galactocentric radius. The thick lines correspond to the average star formation rate surface density for different age intervals (red: 5 < τ < 10 Myr; orange: 50 < τ < 100 Myr; yellow: 400 < Myrτ < 1 Gyr). The shaded areas correspond to the 16th and 84th percentiles. The left Y-axis shows the star formation rate expressed as the total number of stars born; the right Y-axis shows the more conventional amount of stellar mass formed, which requires the adoption of an IMF (here, Kroupa 2001).