Discrete far-infrared (FIR) sources of M31 are identified in the ISO 175um map and characterized via their FIR colours, luminosities and masses. With a mean size of 800pc, they probably represent several clouds in chance projection or giant cloud complexes. The 175um data point provides crucial information in addition to the IRAS 60 and 100um data: At least two modified Planck curves with temperatures of about 40K and 15-21K are necessary to fit the SEDs of the knots. We distinguish between three types of knots - cold, medium, warm - in order to recognize trends. Comparisons with radio and optical tracers show that - statistically - the cold knots can be identified well with CO and HI radio sources and thus might represent mainly molecular cloud complexes. The warm knots coincide with known HII regions and supernova remnants. The medium knots might contain a balanced mixture of molecular clouds and HII regions. The cold knots have a considerable luminosity and their discovery raises the question of hidden star formation. Though the optically dark dust lanes in M31 generally match the FIR ring, surprisingly we do not find a convincing coincidence of our knots with individual dark clouds which might therefore show mainly foreground dust features. The ratio of FIR luminosity to dust mass, L/M, is used to measure the energy content of the dust. The knots have a clear L/M excess over the rest of M31, providing evidence that they are powered by star formation in addition to the interstellar radiation field. The L/M ratio of the warm knots is comparable to that of Galactic HII regions like M42 or NGC2024, while that of the cold knots still reaches values like in the average Orion complex. Thus both the warm and the cold knots are interpreted as containing large cloud complexes with considerable ongoing star formation.
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