COLD DUST IN THE ANDROMEDA GALAXY MAPPED BY ISO

A complete 175 mu m map of the Andromeda galaxy (M31) at 1.'3 resoluti on shows the distribution of cold dust. It is dominated by a ring at 1 0 kpc radius supplemented by a faint outer one at 14 kpc. No clear spi ral pattern is recognisable. The azimuthally averaged radial brightnes s profile is rather flat within the 10 kpc ring and decreases exponent ially 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 f or star formation, as an evolutionary conjecture M31 might be in a tra nsition phase changing its classical optical Sb type spiral morphology towards that of a ringed galaxy. The bulk of the dust has a temperatu re of only 16 K, considerably colder than the 21-22 K previously infer red from the IRAS data and also colder than the 19 K found for the Mil ky Way. The cold dust is accompanied by warm dust, formally described by a component at about 45 K. At the common resolution of 2.'5 the tri plet 60/100/175 mu 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. Th is provides a direct evidence in M31 for the existence of two dust pop ulations - 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 esti mate the corresponding mass from its emission yielding 3.10(7) M., a d ust mass about a factor of ten higher than inferred from the IRAS 60/1 00 mu m 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 tau V approximate to 0.5).