Dust emissivity in the far-infrared

We have derived the dust emissivity in the Far-Infrared (FIR) using data av ailable in the literature. We use two wavelength dependences derived from s pectra of Galactic FIR emission (Reach et al. 1995). A value for the emissi vity, normalised to the extinction efficiency in the V band, has been retri eved from maps of Galactic FIR emission, dust temperature and extinction (S chlegel et al. 1998). Our results are similar to other measurements in the Galaxy but only margin ally consistent with the widely quoted values of Hildebrand (1983) derived on one reflection nebula. The discrepancy with measurements on other reflec tion nebulae (Casey 1991) is higher and suggests a different grain composit ion in these environments with respect to the diffuse interstellar medium. We measure dust masses for a sample of six spiral galaxies with FIR observa tions and obtain gas-to-dust ratios close to the Galactic value.