The complete submillimetre spectrum of NGC 891

Submillimetre maps of NGC 891 have been obtained with the Programme National d'Observations Submillimetriques (PRONAOS) balloon-borne telescope and with the Infrared Space Observatory Photopolarimeter (ISOPHOT) on boardthe ISO satellite. In this article, we also gather data from the Infrared Astronomical Satellite (IRAS) and the Submillimetre Common-User Bolometer Array (SCUBA) to present the complete submillimetre spectrum of this nearby edge-on spiral galaxy. We derive submillimetre emission profiles along the major axis. The modified blackbody fits, assuming a single dust component, lead to temperatures of 19-24 K towards the centre and 18-20 K towards the edges, with possible variations in the dust spectral index from 1.4 to 2. The two-component fits lead to a warm-component temperature of 29 K all along the galaxy, with a cold component at 16 K. The interstellar medium (ISM) masses derived by these two methods are quite different: 4.6 x 10 9 M O . in the case of the one-component model and 12 x 10 9 M O . in the case of the two-component one. This two-component fit indicates that the cold dust to warm dust ratio is between 20 and 40, the highest values being in the wings of this galaxy. Compared to dust mass estimates, both estimates of the ISM mass are consistent with a gas to dust mass ratio of 240, which is close to the Milky Way value. Our results illustrate the importance of accurate submillimetre spectra in deriving masses of the ISM in galaxies.