The emissivity of dust grains in spiral galaxies

We use the radiation transfer simulation of Xilouris et al. (1999) to constrain the quantity of dust in three nearby spiral galaxies (NGC 4013, NGC 5907 and NGC 4565). The predicted visual optical depth from the model is compared with the thermal continuum radiation detected from NGC 4013 and NGC 5907 at 850 mum and from NGC 4565 at 1.2 mm. The former is based on SCUBA images of NGC 4013 and NGC 5907, reduced and presented for the first time in this work. The comparison of visual optical depth and 850 mum (1.2 mm) emission yields the emissivity of dust grains in the submillimeter (millimeter) waveband. We infer a value of 1.2x10(4) for the emissivity at 850 rhom which is a factor 4 higher than the benchmark, semi-empirical model of Draine & Lee (1984). At 1.2 mm our values are a factor 1.5 higher than this model. Our estimates are more closely aligned with recent measurements carried out in the laboratory on amorphous carbon and silicate particulates. A comparison between the distribution of 850 mum (1.2 mm) surface brightness and the intensity levels in the (CO)-C-12(1-0) and 21 cm lines underlines the spatial association between dust detected in the submillimeter/millimeter waveband and molecular Gas clouds. We suggest that the relatively high emissivity values that we derive may be attributable to amorphous, fluffy grains situated in denser gas environments.