We have compared the m emission detected from the nearby, edge-on spiral NGC 891 (Alton et al. 1998) with the corresponding optical depth predicted from a sophisticated radiation transfer model (Xilouris et al. 1998). Morphologically, both model and submm emission show a very similar distribution along the minor axis, with most of the grain mass contained within a 600pc layer (FWHM). Profiles along the major axis show fair agreement between optical extinction and submm radiation. The m emission is shown to trace the bulk of interstellar dust within NGC 891. Therefore, assuming both extinction and submm emission are attributable to the same grains, we can derive the emissivity (or, equivalently, mass absorption coefficient) at m and, thence, the distribution of dust mass along the disk. We believe that this technique is more robust than taking estimates of FIR emissivity for Galactic grains (Hildebrand 1983; Draine and Lee 1984), and extrapolating them to submm wavelengths. The m emissivity we derive for NGC 891 is , which is about a factor of 2-3 higher than the Draine and Lee (1984) model of Galactic dust. Uncertainty in our technique means that the emissivity in NGC 891 could be a factor of 2 higher or lower than the quoted value. Importantly, given that the employed radiation transfer model neglects dust-clumping, a decrease of factor 2 seems quite likely.
We have computed the gas-to-dust ratio along the major axis of NGC 891 using our newly-acquired submm emissivity. We derive a value of 260, which is consistent, within the uncertainties, with estimates for the diffuse ISM of the Milky Way and nearby spirals. The dust in NGC 891 appears to be closely associated with the molecular gas phase, at least for the inner half of the disk. At radii 9 kpc (), it is possible the grains begin to follow the distribution of atomic hydrogen. We use the opacity of the NGC 891 disk to calculate the degree with which light emitted at high redshift is attenuated by foreground field spirals. For galaxies typically associated with the Hubble Deep Field (z=1-2), the fraction of light lost, in this way, is only 5%. Since this estimate is based on a single galaxy as a model, further submm measurements will be required to support our conclusions. In particular, observations at the periphery of spiral disks could establish the extent to which cold dust is associated with the expansive HI envelopes surrounding spiral galaxies (it is the maximum radial extent of the dust which primarily controls the level of foreground extinction). Futhermore, we point out that certain lines-of-sight through nearby clusters are likely to be much more optically thick (than our figure of 5% would suggest), particularly if grains are expelled into the intergalactic medium via starburst winds (Alton et al. 1999c) or dispersed through tidal interactions (Yun et al. 1994).
Finally, we note the strong correspondance between dust and molecular gas in NGC 891. Further submm imaging of face-on spirals (both H2 rich and H2 poor) would confirm the trend indicated by the current observations, that the bulk of galactic dust appears to be associated with the molecular gas phase.
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000