  A spiral galaxy partially ovelapping a more distant elliptical offers an
unique opportunity to measure the dust extinction in the foreground spiral.
  From the Sloan Digital Sky Survey 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 to determine 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, higher resolution than the SDSS
images is needed to disentangle the effects of spiral arms and Hubble type from
evolution since z = 0.2.

Astrophysics