Structure and Evolution of the Opacity of Spiral Disks

The opacity of a spiral disk due to dust absorption influences every measurement we make of it in the UV and optical. Two separate techniques directly measure the total absorption by dust in the disk: calibrated distant galaxy counts and overlapping galaxy pairs. The main results from both so far are a semi-transparent disk with more opaque arms, and a relation between surface brightness and disk opacity. In the Spitzer era, SED models of spiral disks add a new perspective on the role of dust in spiral disks. Combined with the overall opacity from galaxy counts, they yield a typical optical depth of the dusty ISM clouds: 0.4 that implies a size of∼ 60 pc. Work on galaxy counts is currently ongoing on the ACS fields of M51, M101 and M81. Occulting galaxies offer the possibility of probing the history of disk opacity from higher redshift pairs. Evolution in disk opacity could influence distance measurements (SN1a, TullyFisher relation). Here, we present first results from spectroscopically selected occulting pairs in the SDSS. The redshift range for this sample is limited, but does offer a first insight into disk opacity evolution as well as a reference for higher redshift measurements. The opacity of spiral disks is a characteristic that influences many of our observations of disks at any redshift. Thus far, two observational methods have produced reliable measurements of disk opacity; occulting galaxy pairs and the calibrated number of more distant galaxies. Together with Spitzer SED models, the apparent optical depth can be used to estimate general properties of the extincting ISM ’s structure. 1. Distant Galaxy Counts The number of distant galaxies seen in an HST image through the face-on foreground spiral is a direct indication of its opacity, after proper calibration using artificial galaxy counts (synthetic field method (SFM) González et al. 1998; Holwerda et al. 2005a). We have obtained calibrated galaxy counts for a sample of 32 deep HST/WFPC2 fields. The main results from the disk opacity study are: (1) most of the disks are semitransparent (Holwerda et al. 2005b, Figure 1), (2) arms are more opaque (Holwerda et al. 2005b), (3) as are brighter sections of Space Telescope Science Institute