RESOLVED 200-MU-M IMAGES OF NEARBY GALAXIES - EVIDENCE FOR AN EXTENDED DISTRIBUTION OF COLD DUST

e present resolved 200 mu m images for 8 nearby galaxies observed with the Infrared Space Observatory (ISO). By comparing the 200 mu m obser vations with IRAS 60 mu m and 100 mu m data, we find that cold dust be comes more dominant at larger radii. We infer a grain temperature of 1 8-21 K for this cold component i.e. about 10 K lower than the warm dus t detected by IRAS in external spirals. This value is close to theoret ical predictions in the literature based on heating by the general int erstellar radiation field. A comparison of the 200 mu m images with co mplementary 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 ma gnitude higher than the value infered for the Galaxy. By analysing the 200 mu m data for our sample, we derive a mean gas-to-dust ratio of s imilar to 225 which is close to the value in the solar neighborhood (1 50-300). It is likely that IRAS may have 'overlooked' the vast majorit y of grains residing in spiral disks.