Cold dust in the Andromeda Galaxy mapped by ISO ?

A complete 175 μm 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 recognisable. 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, considerably 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 resolution of 2.′5 the triplet 60/100/175 μm 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 3·107 M , a dust mass about a factor of ten higher than inferred from the IRAS 60/100 μm 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 τV ≈ 0.5).