Cold dust in the Andromeda Galaxy mapped by ISO

A complete 175m map of the Andromeda galaxy (M31) at 1: 3 resolution shows the distribution of cold dust. It is dominated by a ring at 10 kpc radius supplemented by a faint outer one at 14 kpc. No clear spiral pattern is recognis- able. The azimuthally averaged radial brightness profile is rather flat within the 10 kpc ring and decreases exponentially outside thereof, discernible down to a brightness of 0.07 MJy/sr at a distance of 22 kpc. Since the ring comprises a large reservoir for star formation, as an evolutionary conjecture M31 might be in a transition phase changing its classical optical Sb type spiral morphology towards that of a ringed galaxy. The bulk of the dust has a temperature of only 16 K, con- siderably colder than the 21-22 K previously inferred from the IRAS data and also colder than the 19 K found for the Milky Way. The cold dust is accompanied by warm dust, formally described by a component at about 45 K. At the common res- olution of 2: 5 the triplet 60/100/175 m flux ratio varies only little across the rings as well as the disk, thus everywhere in M31 at least two dust components are required to fit the far-infrared spectral energy distribution. This provides a direct evidence in M31 for the existence of two dust populations - small and large grains - similar to what had been found in the Milky Way. For the cold dust component around 16 K we can now estimate the corresponding mass from its emission yielding 310 7 M, a dust mass about a factor of ten higher than in- ferred from the IRAS 60/100m data alone. The new cold dust mass - if evenly distributed in the plane of the galaxy - would be sufficient to make the disk of M31 moderately opaque in the optical (face-on: 0.1 M pc 2 corresponding to V 0.5).