Dust heating maps of a face-on view of M 51, as obtained from the 3D dust cell data. Left panel: dust heating fraction by the old stellar population; middle panel: dust heating fraction by the young stellar populations; and right panel: dust heating fraction by M 51b. The rightmost map is shown on a log-scale, and the others are on a linear scale. The inner and outer green circles indicate the 3 kpc and 8 kpc radii, respectively. The red cross is the centre of M 51b. The gold circle indicates the extent of the dusty disc of M 51b at 2 kpc radius. All maps share a the same pixel scale as the PACS 160 μm image: 4.0″.