Dust in spiral galaxies: Comparing emission and absorption to constrain small-scale and very cold structures

The detailed distribution of dust in the disks of spiral galaxies is important to understanding the radiative transfer within disks and to measuring overall dust masses if significant quantities of dust are either very opaque or very cold. We address this issue by comparing measures of dust absorption, using the galaxy-overlap technique in the optical, with measures of the dust grains' thermal emission from 50-2000 mu m, using ISOPHOT on board Infrared Space Observatory (ISO) and SCUBA at the James Clerk Maxwell Telescope. We examine three spiral galaxies projected partially in front of E/S0 galaxies: AM 1316-241, NGC 5545, and NGC 5091 (for NGC 5091 we have only optical and ISO data). Adopting an empirical exponential model for the dust distribution, we compare column densities and dust masses derived from the absorption and emission techniques. This comparison is sensitive to the amount of dust mass in small, opaque structures, which would not contribute strongly to area-weighted absorption measures, and to very cold dust, which would contribute to optical absorption but provide only a small fraction of the submillimeter emission. In AM 1316-241, we find global dust masses of 2-5 x 10(7) M., with the two techniques agreeing at the 50% level. NGC 5545 has about half this dust mass. The concordance of dust masses is well within the errors expected from our knowledge of the radial distribution of dust and argues against any dominant part of the dust mass being so cold or opaque. The 50-2000 mu m data are well fitted by modified Planck functions with an emissivity law beta = -2, at 21 +/- 2 K; a modest contribution from warmer dust is required to fit only the 50 mu m measurement of NGC 5545. We incorporate empirical corrections to the flux scale of ISOPHOT P32 data, which can reach a factor 2 from comparison of IRAS and ISO fluxes for objects in two programs. We also present 12 mu m ISOCAM observations of these pairs. The light profiles at this wavelength exhibit shorter disk scale lengths than in the optical. Comparison of H alpha and 12 mu m images of NGC 5545 indicate that ISOCAM images are reliable tracers of star formation.