Resolved 200μm images of nearby galaxies - Evidence for an extended distribution of cold dust

We present resolved 200μm images for 8 nearby galaxies observed with the Infrared Space Observatory (ISO). By comparing the 200μm observations with IRAS 60μm and 100μm data, we find that cold dust becomes more dominant at larger radii. We infer a grain temperature of 18-21 K for this cold component i.e. about 10 K lower than the warm dust detected by IRAS in external spirals. This value is close to theoretical predictions in the literature based on heating by the general interstellar radiation field. A comparison of the 200μm images with complementary B-band data also shows that the cold dust is radially more extensive than the stars. The gas-to-dust ratio of external spirals, derived using IRAS fluxes, has been claimed to be about an order of magnitude higher than the value infered for the Galaxy. By analysing the 200μm data for our sample, we derive a mean gas-to-dust ratio of ∼ 225 which is close to the value in the solar neighborhood (150-300). It is likely that IRAS may have 'overlooked' the vast majority of grains residing in spiral disks.