The azimuthally averaged profiles of the “cool” and the “warm” dust luminosities (green and blue lines) in units of L_⊙   pc^-2. The cool-to-warm luminosity ratio is also plotted (red line). The luminosities are calculated for the whole galaxy (upper panel) and the disk of the galaxy only (middle panel; see Sect. 6.2). The residuals between the two profiles give the radial distribution of the luminosity produced by the spiral arm network (bottom panel).