IMAGING IN THE OPTICAL AND NEAR-INFRARED REGIMES .2. ARCSECOND SPATIAL-RESOLUTION OF WIDELY DISTRIBUTED COLD DUST IN SPIRAL GALAXIES

For extragalactic systems, the InfraRed Astronomical Satellite IRAS on ly surveyed 'warm' dust; in spirals, that dust delineates spiral arms and is associated with OB stars, H II and star forming regions. On the other hand, sub-mm/mm observations of spiral galaxies have brought th e issue of cold dust to the fore, but with controversial and diametric ally opposed conclusions. The uncertainty in the estimate of the avera ge far- IR to mm flux ratio amounts to one order of magnitude or more. A key problem lies in our current lack of knowledge of the spatial ex tension of cold dust. We contend that high spatial resolution, ground- based, digital imaging in the optical and near-infrared bands is sensi tive to dust grains of all dust temperatures; moreover, the resolution in such dust tracing surveys is one order of magnitude better than th at possible with the largest sub-mm/mm telescopes and two orders of ma gnitude better than with IRAS. We demonstrate that radiative transfer models (involving multiple scattering) combined with B - K' and V - K' colour maps, provide a powerful and independent tool for exploring th e widespread distribution of cold interarm dust powered by the general interstellar radiation field of old disk stars. Two galaxies (NGC 473 6 and NGC 4826), both detected in the sub-mm, are selected to demonstr ate the technique for an embedded dust geometry and that of a foregrou nd screen. We identify widely distributed interarm cirrus dust embedde d within the disk of NGC 4736, and we compare extinctions using our te chnique with extinctions inferred from far-infrared observations secur ed with the Kuiper Airborne Observatory. For NGC 4826, the most likely dust geometry is that of a distinct, foreground dust screen. The powe r of the new method's quantitative accuracy is that, by relying on rad iative transfer models invoking continuous media, the cold dust masses we infer are lower limits. The transmission coefficient in clumpy, he terogeneous, N-phase scattering ISMs will be even higher. As a lower l imit, NGC 4736 and NGC 4826 require an additional 90 per cent in the f orm of cold dust - a dust component essentially missed by IRAS. An imp ortant conclusion from our work is that the Galactic dust-to-gas ratio , rather than being exceptionally high, may again be rather representa tive - at least for the spiral galaxies studied here.