Spatially resolved images of NGC 628. Top left: SDSS g-band image of NGC 628 with concentric annuli of 36 arcsec width. Top right: Dust-mass surface density of the whole disk. The color level indicates the dust surface density in units of M_⊙ pc⁻² as shown in the color bar. Bottom left: S/N map corresponding to the minimum S/N of all the observed fluxes in each pixel. Bottom right: Total 350 μm flux for the low-S/N pixels pixels with S/N > 2 in the SPIRE 350 μm image with respect to the total flux from the low-S/N pixels in each ring. The same figure is shown in Appendix A for the other galaxies in the sample.