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, spectral energy distribution models of spiral disks add a new perspective on the role of dust in spiral disks. Combined with the overall opacity from galaxy counts, we can derive a typical optical depth of the dusty interstellar medium clouds: 0.4 which implies a size of ∼ 60 pc. Current work on galaxy counts is in the ACS fields of M 51, M 101 and M 81. Occulting galaxies offer the possibility of probing the history of disk opacity from higher redshift occulting pairs. Evolution in disk opacity could influence distance measurements (SN1a, Tully-Fisher relation). Here, we present first results from spectroscopically selected occulting pairs in the Sloan Digital Sky Survey. 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.
Astronomical Society of the Pacific Conference Series