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Spiral Disk Opacity from Occulting Galaxy Pairs in the Sloan Digital Sky Survey

A spiral galaxy partially overlapping a more distant elliptical offers a unique opportunity to measure the dust extinction in the foreground spiral. From the Sloan Digital Sky Survey (SDSS) DR4 spectroscopic sample, we selected 83 occulting galaxy pairs and measured disk opacity over the redshift range z = 0.0-0.2 with the goal of determining the recent evolution of disk dust opacity. The enrichment of the ISM changes over the lifetime of a disk, and it is reasonable to expect the dust extinction properties of spiral disks as a whole to change over their lifetime. When they do, the change will affect our measurements of galaxies over the observable universe. From the SDSS pairs we conclude that spiral disks show evidence of extinction to ∼2 effective radii. However, no evidence for recent evolution of disk opacity is evident, due to the limited redshift range and our inability to distinguish other factors on disk opacity such as the presence of spiral arms and Hubble type. Such effects also mask any relation between surface brightness and optical depth that has been found in nearby galaxies. Hence, we conclude that the SDSS spectral catalog is an excellent way to find occulting pairs and construct a uniform local sample. However, a higher resolution than that of the SDSS images is needed to disentangle the effects of spiral arms and Hubble type from evolution since z = 0.2.