Various radial profiles of M 51 obtained from the 3D dust cell data. First column, from top to bottom: radial profiles of the dust heating fractions and the dust temperature for M 51a. The vertical green lines indicate the 3 kpc and 8 kpc radii. Second column, top: radial profile of f_M 51b up to 20 kpc radii from the centre of M 51b; the blue line gives the location of the central region of M 51a. Second column, bottom: radial profile of the dust temperature of M 51b up to a radius of 2 kpc radii; the dashed lines passing though the points represent the running median. The points are colour-coded according to the same quantity as in the y-axis. The level of transparency indicates the point density.