The bright 175 mu m knots of the Andromeda galaxy

Discrete far-infrared (FIR) sources of M 31 are identified in the ISO 175 m um map and characterized via their FIR colours, luminosities and masses in order to reveal the nature of these knots. With our spatial resolution of 3 00 pc at M 31's distance, the FIR knots arc:clearly seen as extended object s with a mean size of about 800 pc. Since this appears too large for a sing le dust cloud, the knots might represent several clouds in chance projectio n or giant cloud complexes. The 175 mum data point provides crucial information in addition to the IRAS 60 and 100 mum data: At least two lambda (-2) modified Planckian curves wi th temperatures of about 40 K and 15-21 K are necessary to fit the spectral energy distributions (SEDs) of the knots. Though they show a continuous ra nge of temperatures, we distinguish between three types of knots - cold, me dium, warm - in order to recognize trends. Comparisons with radio and optic al tracers show that - statistically - the cold knots can be identified wel l with CO and HI radio sources and thus might represent mainly molecular cl oud complexes. The warm knots coincide with known H II regions and supernov a remnants. The medium knots might contain a balanced mixture of molecular clouds and HII regions. The cold knots have a considerable luminosity and t heir discovery raises the question of hidden star formation. Though the optically dark dust lanes in M 31 generally match the FIR ring, surprisingly we do not find a convincing coincidence of our knots with indi vidual dark clouds, which might therefore show mainly foreground dust featu res. The ratio of FIR luminosity to dust mass, Llnl, is used to measure the ener gy content of the dust. It can originate from both the interstellar radiati on held and still embedded stars recently formed. 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. Furthermore , the L/M ratio of the warm knots is comparable to that of Galactic Hn regi ons like M 42 or NGC 2024, while that of the cold knots still reaches value s like in the average Orion complex. Thus both the warm and even the cold k nots are interpreted as containing large cloud complexes with considerable ongoing star formation.