INTERSTELLAR DUST IN GALAXIES

A new model for interstellar grains is presented, based on a revised version of the model of Rowan-Robinson. The main new features are the incorporation of very small grains (a = 5-20 angstrom) to explain the diffuse mid-infrared emission and the addition of very large grains (a = 30 mum) to explain excess emission at millimetre wavelengths. The model provides an excellent fit to the interstellar extinction curve and to the far-infrared spectrum of dust in our Galaxy. The model is compared to observed far-infrared colour-colour diagrams for galaxies, both for those galaxies unresolved by IRAS and for nearby, resolved galaxies. The far-infrared spectra of the large, nearby spirals mapped by IRAS, for which the emission is predominantly cirrus, are modelled in detail. The inferred dust extinctions averaged over the galactic discs are found to be small, A(V) = 0.006-0.1, though the values in the central 40 per cent of the galaxies, where the bulk of the far-infrared emission is produced, are likely to be a factor of 6 or so higher. The concept of optically thick spiral galaxies advocated by Disney, Davies & Phillips and Valentijn is not supported by this work. Models are also given for the infrared-to-submillimetre spectra of galaxies which have been mapped at 800 mum, incorporating starburst and cirrus models. The starburst component also appears to require the 30-mum grain component. The accuracy of the dust masses derived from modelling far-infrared spectra is enormously improved if observations at long wavelengths are available.