200-μm ISO observations of NGC 6946: evidence for an extended distribution of cold dust

We have used the Infrared Space Observatory (ISO) to obtain a resolved 200-μm image of the nearby spiral galaxy NGC 6946. We compare these observations with those made at 60 and 100 μm by the Infrared Astronomical Satellite (IRAS). Whereas IRAS mainly detected warm dust (T ~ 30 K) heated by star-forming regions, ISO detects an extended distribution of cold dust (T <= 22 K) that is heated by the general interstellar radiation field. The IRAS emission tends to follow the molecular gas while it appears that the colder dust has a stronger association with atomic hydrogen. The cold dust is more extended in the radial direction than the optical, near-infrared, IRAS and radio continuum (20 cm) emission. The total gas-to-dust mass ratio is found to be ~ 290. This is much closer to the Galactic value of ~ 160 than the value of 1240 found using IRAS data only