2000MNRAS.312L...5S

Submillimetre mapping observations of the active edge-on spiral galaxy NGC 3079 are presented. These maps at 850 and 450µm were made with the Submillimetre Common User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT).

The source structure at these wavelengths consists of a central unresolved source embedded in diffuse disc emission, similar to that displayed at 1.2mm. The disc emission is fitted with two optically thin, isothermal dust models which give temperatures of 12 and 31K, similar to those derived previously by Braine et al. The core component is well described by a single-temperature fit (∼32K). The combined dust mass from these observations, using the same mass absorption coefficient as is 3.5x10⁸M_☉, of which ∼90per cent resides in the cold component of the galactic disc. The effect of the cold dust component detected by SCUBA is thus to reduce the global gas-to-dust mass ratio from ∼1400 found in the above study to 85, very similar to the Galactic level. Calculations using the models of Draine & Lee and/or alternative molecular gas mass estimates yield gas-to-dust mass ratios in the range 60-190.

The data presented here, together with previously published 1.2-mm mapping observations and IRAS data, are inconsistent with detections made with the Infrared Space Observatory (ISO). In particular, the latter give an excess of flux at 200 and 180µm relative to that predicted by our simple model fits (approximately a factor of 2-3).