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 = E _ { B - V } / A _ { V } = 3.4 \pm 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 \approx 2.5 \pm 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 can not 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 shows only some aspects of a fractal nature .
In neither galaxy do we see significant regions even on single-pixel scales in spiral arms with A _ { B } > 2.5 .
The measurements in NGC 1275 are compromised by our lack of independent knowledge of the foreground system ’ s light distribution , but masked sampling of the absorption suggests an effective reddening curve much flatter than the Milky Way mean ( but this may indicate that the foreground system has been affected by immersion in the hot intracluster gas , or is inside the stellar distribution of NGC 1275 ) .
The bright blue star clusters trace the absorption in this system quite closely , indicating that these clusters belong to the foreground system and not to NGC 1275 itself .