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W.C. Keel, R.E. White III (U. Alabama)
HST images of overlapping-galaxy pairs offer new insight into the fine structure of dust in spiral disks. We present results from four such pairs. For AM1316-241, we find that the distribution of dust is approximately scale-free (i.e., fractal) over a range of about 1.5 dex in linear scale, and the color-color behavior of extinction in individual pixels indicates that the lower limit of this range (\approx 150 pc) is physically significant (which appears in AM0500-620 as well). The pixel distribution of extinction in B and I passbands follows a Galactic screen law closely, so that dust structure within each pixel cannot have a very large amplitude in column density. The foreground disk system in NGC 1275 is larger and brighter than simple inspection of typical images suggests, including most of the absorbing material seen near the nucleus of NGC 1275. The dust lanes show extinction up to AB=0.55. The association of bright clusters with the dust lanes lends weight to the interpretation of this system as a late-type spiral. The spiral/spiral superposition NGC 3314 reveals new aspects of dust in the foreground system, since almost the entire foreground disk is backlit. The intensity pattern in dust lanes crossing the edge of the background disk indicates that dust lanes can have a larger scale height than the disk starlight does. The spiral pattern in this disk is defined as much by dust as by luminous blue stars. Detection of the background nucleus in NGC 3314 in I (and in an IRTF K image) allows an estimate of the extinction along a line of sight passing only about 300 pc from the center of the foreground Sc (at about 0.1 R25). For the reddest plausible color of the background bulge light, the interarm extinction at this radius is of order AI=2.7 magnitudes, implying AB=4.6. The spiral dust lanes at this radius have optical depths at least three times these values, since we detect no transmitted I light. These values are consistent with a simple model based on observations in outer disks of several spirals, and give an anchoring point for modelling the inner regions as well. This work was supported by NASA through STScI grant GO-06438.01-95A.