AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 7. Spirals and Ellipticals
Display, Monday, May 31, 1999, 9:20am-6:30pm, Southwest Exhibit Hall

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[7.04] Dust in Spiral Galaxies: Comparing Emission and Absorption to Constrain Small-Scale and Very Cold Structures

D.L. Domingue, W.C. Keel, R.E. White,III (U. Alabama), S.D. Ryder (JAC)

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 either very opaque or very cold dust structures are important. We address this issue by comparing measures of dust absorption, using the overlap technique in the optical, with measures of the grains' thermal emission from 50-2000 \mum using ISOPHOT and SCUBA at the JCMT. We examine two spirals projected partially in front of E/S0 galaxies - AM1316-241 and NGC 5545. Adopting an empirical exponential model for the dust distribution, we compare column densities and dust masses from the two very different techniques. This comparison is sensitive to the amount of dust mass in small, opaque structures, which would not strongly affect area-weighted absorption measures, and to very cold dust, which would contribute to optical absorption but provide only a small fraction of the submm emission. In AM1316-241, we find global dust masses 2-5 \times 107 solar masses, with both techniques matching within 50%. NGC 5545 has about half this dust mass, with the absorption data in this case less well-constrained. 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 in such hidden forms.

The 50-2000 \mum data are well fitted by modified Planck functions with emissivity law \beta=-2, at 21 ±2 K, with a modest contribution from warmer dust required to fit only the 50\mum 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.

This work was supported by NASA/ISO grant NAG 5-3336.

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