Relation between sSFR and f_young. The blue points represent the dust cells that belong to M 51a within a 10 kpc radius of its centre, and the orange points the dust cells of M 51b. The level of transparency indicates the points density. The red line is the fit through the points of M 51a with Eq. (7). The purple diamonds are the pixel values of M 51a, and the purple line is the power-law fit to that data from the model of De Looze et al. (2014). The black line shows the MS of galaxies as derived in Nersesian et al. (2020) by fitting the voxels of the radiative transfer models of six face-on late-type galaxies.