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A great deal of attention has been devoted to the characterization of nearby spirals' IR emission. This has led to a deeper understanding of the distribution of dust in galaxies.

2.1. Cold dust and the total amount of dust

Assuming that the IRAS 60 and 100 µm fluxes from galaxies sample a single dust phase, dust masses and temperatures have been computed. Typical dust temperatures of ~ 30K and dust masses one order of magnitude below that deduced for the Milky Way were obtained (e.g. [Devereux & Young1990]). The advent of millimeter bolometers showed that this was likely an artifact of the wavelength domain sampled by IRAS, but uncertainties in the exponent of the emissivity in the FIR/mm range prevented a definitive assessment of the question.

With the combination of ISO and SCUBA, this is now being resolved. The full FIR/mm SEDs of nearby spiral galaxies reveal a consistent picture: in normal galaxies (i.e. star-forming but not starbursting spirals) most of the emission longward of 150-200 µm is provided by a cold dust phase (T appeq 10-20K, [Alton et al. 1998a]; [Bianchi et al. 1998]; [Davies et al. 1999]; [Haas et al. 1998]; [Israel et al. 1999]; [Krügel et al. 1998]). In fact, Alton et al. 1998a show clearly that the cold dust emission provides a significant fraction of the 100 µm flux. This mixed nature of the 100 µm IRAS band is very likely at the origin of numerous controversies on the interpretation of IRAS luminosities. Dust masses measurements now yield gas-to-dust ratios in external galaxies in the range 100-300, much more in agreement with the value of ~ 160 measured in our galaxy.

These studies also point to a FIR/mm emissivity index of 2 (i.e. beta in kappanu propto nubeta) rather than the more commonly used 1.5.

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