Seeing Galaxies Through Thick & Thin. III. HST Imaging of the Dust in Backlit Spiral Galaxies

We present analysis of WFPC2 imaging of two spiral galaxies partially backlit by E/S0 systems in the pairs AM1316-241 and AM0500-620, as well as the (probably spiral) foreground system in NGC 1275. Images in B and I are used to determine the reddening curve of dust in these systems. The foreground spiral component of AM1316-241 shows dust strongly concentrated in discrete arms, with a reddening law very close to the Milky Way mean (R = EB−V /AV = 3.4 ± 0.2). The dust distribution is scale-free between about 100 pc and the arm dimension, about 8 kpc. The foreground spiral in AM0500-620 shows dust concentrated in arms and interarm spurs, with measurable interarm extinction as well. In this case, although the dust properties are less well-determined than in AM1316-241, we find evidence for a steeper extinction law than the Milky Way mean (formally, R ≈ 2.5 ± 0.4, with substantial variation depending on data quality in each region). The shape of the reddening law suggests that, at least in AM1316-241, we have resolved most of the dust structure. In AM0500-620 it is less clear that we have resolved most of the dust structure, since the errors are larger. In AM0500-620, the slope of the perimeter-scale relation (associated with fractal analysis) steepens systematically on going from regions of low to high extinction. A perimeter-smoothing length test for scale-free (fractal) behavior in AM1316-241 shows a logarithmic slope typically −0.4 on 100-1000 pc scales. However, we cannot determine a unique fractal dimension from the defining area-perimeter relation, so the projected dust distribution is best defined as fractal-like. For scales above 2-4 pixels (120-250 pc), the box-counting estimate gives a fractal dimension close to 1.4, but the perimeter-area relation gives a dimension of 0.7 on large scales and inconsistent results for small scales, so that the distribution