Left: stellar mass map of M 51, shown on a scale that emphasises the oversubtractions associated with the ICA dust correction; the red contours are the regions of M 51 where the [3.6]–[4.5] colour is very red, basically corresponding to HII regions ([ 3.6 ]−[ 4.5 ] > 0.2). The blue circles mark the positions of field stars, with the clear signature of concentric circles in the colour map due to a saturated PSF. Middle: [3.6]–[4.5] colour map of M 51; compare the lack of very red regions in the central R< 22′′ ~ 1 kpc with their presence in the spiral arms. Right: one of the 40 random realisations of the model with oversubtractions as described in the text (Sect. A.1), which we use to assess the effect of these imperfections on gravitational torques.