The opacity of spiral galaxy disks.: VIII.: Structure of the cold ISM

The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the extinction of a known source. In this paper we compare two techniques, one based on emission and one on absorption, applied to sections of 14 disk galaxies. The two measurements reflect, respectively, the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk. The small-scale geometry of the cold ISM is critical for accurate models of the overall energy budget of spiral disks. ISM geometry, relative contributions of different stellar populations, and dust emissivity are all free parameters in galaxy spectral energy distribution ( SED) models; they are also sometimes degenerate, depending on wavelength coverage. Our aim is to constrain the typical ISM geometry. The apparent optical depth measurement comes from the number of distant galaxies seen in Hubble Space Telescope (HST) images through the foreground disk, calibrated with the synthetic field method (SFM). We discuss what can be learned from the SFM measurement alone regarding ISM geometry. We measure the IR flux in images from the Spitzer Infrared Nearby Galaxy Survey in the same section of the disk that was covered by HST. A physical model of the dust is fit to the SED to estimate the dust surface density, mean temperature, and brightness in these disk sections. The surface density is subsequently converted into the average optical depth estimate. The two measurements generally agree, and the SED model finds mostly cold dust ( T < 25 K). The ratios between the measured average and apparent optical depths of the disk sections imply optically thin (tau(c =) 0.4) clouds in these disks. Optically thick disks are likely to have more than a single cloud along the line of sight.